Make FILE a macro that expands to FILE.

This solves a compatibility problem with libgmp that erroneously attempts to
detect whether <stdio.h> has been included, but doesn't know what the include
guards for Sortix libc are.

libc/decl/FILE.h

@@ -1,4 +1,5 @@
 #ifndef _FILE_DECL
 #define _FILE_DECL
 typedef struct FILE FILE;
+#define FILE FILE
 #endif