Fix Clang warning about undeclared function in mmap test

Clang on Linux complains that "creat" isn't declared. According to
POSIX, the declaration is in fcntl.h.

Issue: eng/toolchain/gnatcoll-core#62

testsuite/tests/mmap/c_support.c

@@ -2,6 +2,7 @@
 #include <stdio.h>
 #include <unistd.h>
 #include <sys/stat.h>
+#include <fcntl.h>
 
 /* We don't use standard lseek implementation on Windows otherwise
  * creating very large file will be inefficient as mingw lseek will