LibC: Define TMP_MAX

This is required by POSIX and is supposed to be equal to the number
of unique filenames that tmpnam can return. Given that we only have a
stub for that function, TMP_MAX is only here to please libcxx tests.

Userland/Libraries/LibC/stdio.h

@@ -21,6 +21,7 @@
 
 #define FILENAME_MAX 1024
 #define FOPEN_MAX 1024
+#define TMP_MAX 1000
 
 __BEGIN_DECLS
 #ifndef EOF