kbuild: modpost, corrected check of mmap()

From: Kristian Høgsberg <krh@bitplanet.net>

Corrected typo in modpost.c. Check the pointer returned, not the pointer
to the mmap funtion

scripts/modpost.c

@@ -193,7 +193,7 @@ grab_file(const char *filename, unsigned long *size)
 
 	*size = st.st_size;
 	map = mmap(NULL, *size, PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, 0);
-	if (mmap == MAP_FAILED) {
+	if (map == MAP_FAILED) {
 		perror(filename);
 		abort();
 	}