v3: avoid duplicate Mach timebase extern

Author: medvednikov

vlib/v3/gen/c/cleanc.v

@@ -17957,6 +17957,7 @@ const c_headerless_libc_declared_fns = [
 	'__errno',
 	'__errno_location',
 	'_errno',
+	'mach_timebase_info',
 	'ptrace',
 	'sigaddset',
 	'sigprocmask',

vlib/v3/gen/c/preamble_test.v

@@ -50,6 +50,12 @@ fn test_headerless_libc_preamble_declares_qsort_for_generated_sort_helpers() {
 	assert c_code.contains('void qsort(void* base, size_t items, size_t item_size, int (*cb)(const void*, const void*));'), c_code
 }
 
+fn test_headerless_libc_preamble_suppresses_its_mach_timebase_declaration() {
+	mut g := FlatGen.new()
+	g.headerless_libc_preamble()
+	assert !g.should_emit_c_extern_decl('mach_timebase_info')
+}
+
 fn test_headerless_platform_constants_include_process_errno_values() {
 	mut g := FlatGen.new()
 	g.headerless_platform_constants()