x86: include header to define types in use

The tsc header is using u32 w/o including the file
with defines it.

Change-Id: I9fcad882d25e93b4c0032b32abd2432b0169a068
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/4843
Tested-by: build bot (Jenkins)
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>

src/include/cpu/x86/tsc.h

@@ -1,6 +1,8 @@
 #ifndef CPU_X86_TSC_H
 #define CPU_X86_TSC_H
 
+#include <stdint.h>
+
 #if CONFIG_TSC_SYNC_MFENCE
 #define TSC_SYNC "mfence\n"
 #elif CONFIG_TSC_SYNC_LFENCE