Replace stddef.h by string.h in win32 platform specifics

For some reasons, <corecrt.h> fails to compile on my machine, but <stddef.h> fails on the C.I.
The stack of uncompatible macros is getting really fragile...
This commit is only a wild guess.
IMO we should try and follow standards where possible.

Author: nicolas-cellier-aka-nice

platforms/win32/vm/sqPlatformSpecific.h

@@ -24,7 +24,7 @@
 
 #if defined(__MINGW32__) || defined(__MINGW64__)
 // vcruntime.h defines size_t for MSVC builds, but not for mingw builds.
-# include <stddef.h> // for size_t
+# include <string.h> // for size_t
 #endif
 
 /* File positions in the FilePlugin & other plugins */