gmappedfile.c: Define S_IFREG on Win32 when it's not available

Make a workaround for S_IFREG in gmappedfile.c on Windows as it may not be
available for a given compiler on that platform

glib/gmappedfile.c

@@ -41,6 +41,10 @@
 #define fstat(a,b) _fstati64(a,b)
 #define stat _stati64
 
+#ifndef S_ISREG
+#define S_ISREG(m) (((m) & _S_IFMT) == _S_IFREG)
+#endif
+
 #endif
 
 #include "gconvert.h"