builtin: fix GetFinalPathNameByHandleW declaration for TCC on Windows

TCC''s bundled windows.h does not include fileapi.h, so
GetFinalPathNameByHandleW had no C declaration in scope when TCC
compiled any program that pulled in builtin. TCC (in C99 mode) treats
implicit function declarations as hard errors.

Use a compiler-conditional declaration:

- Under TCC, mark it @[c_extern] so V emits a forward declaration
  (guarded by #ifndef). TCC''s fileapi.h cannot be included directly
  because it pulls in apiset.h, which TCC does not bundle, so supplying
  the declaration from V is the only viable route.
- Under GCC/MSVC, declare it bare (no @[c_extern]) so V emits no
  declaration and the call resolves against the SDK header that
  windows.h pulls in transitively. Emitting a V extern here would
  conflict with the SDK''s `DWORD WINAPI` signature (DWORD is
  unsigned long, not u32) and fail with -Werror.

The whole block is guarded by $if windows {} so the symbol is not
compiled into Linux/macOS builds, where it does not exist.

Verified: generated C contains the extern only for the TCC backend and
omits it for GCC; `vlib/os/` tests pass (24/24) on Windows with GCC.

Co-Authored-By: WOZCODE <contact@withwoz.com>

Author: PythonWillRule

vlib/builtin/cfns.c.v

@@ -295,7 +295,18 @@ fn C.CreateFile(lpFilename &u16, dwDesiredAccess u32, dwShareMode u32, lpSecurit
 fn C.CreateFileW(lpFilename &u16, dwDesiredAccess u32, dwShareMode u32, lpSecurityAttributes &u16, dwCreationDisposition u32,
 	dwFlagsAndAttributes u32, hTemplateFile voidptr) voidptr
 
-fn C.GetFinalPathNameByHandleW(hFile voidptr, lpFilePath &u16, nSize u32, dwFlags u32) u32
+$if windows {
+	$if tinyc {
+		// TCC's bundled windows.h does not include fileapi.h (which itself
+		// pulls in apiset.h that TCC lacks), so the function has no
+		// declaration in scope; @[c_extern] makes V emit one without
+		// conflicting with the SDK header that GCC/MSVC pull in transitively.
+		@[c_extern]
+		fn C.GetFinalPathNameByHandleW(hFile voidptr, lpFilePath &u16, nSize u32, dwFlags u32) u32
+	} $else {
+		fn C.GetFinalPathNameByHandleW(hFile voidptr, lpFilePath &u16, nSize u32, dwFlags u32) u32
+	}
+}
 
 fn C.CreatePipe(hReadPipe &voidptr, hWritePipe &voidptr, lpPipeAttributes voidptr, nSize u32) bool