[vm/ffi] dart_api_dl.c fix MacOS build

Fixes "redefinition of typedef '...' is a C11 feature" on Mac bots.

Change-Id: I9f1114279106fa83ca491e069564a9931541d6ad
Cq-Include-Trybots:dart/try:vm-kernel-mac-release-x64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/158001
Auto-Submit: Daco Harkes <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>
Reviewed-by: Alexander Aprelev <[email protected]>

Author: dcharkes

runtime/include/dart_api_dl.c

@@ -10,9 +10,7 @@
 
 #include <string.h>
 
-#define DART_API_DL_DEFINITIONS(name, R, A)                                    \
-  typedef R(*name##_Type) A;                                                   \
-  name##_Type name##_DL = NULL;
+#define DART_API_DL_DEFINITIONS(name, R, A) name##_Type name##_DL = NULL;
 
 DART_API_ALL_DL_SYMBOLS(DART_API_DL_DEFINITIONS)