commonlib/include: Include <stdint.h> to fix 'SIZE_MAX' undeclared error

This change includes the <stdint.h> header file to resolve the
compilation error "'SIZE_MAX' undeclared". This issue was introduced
by commit hash af0d4bce65df277b56e495892dff1c712ed76ddd (region:
Introduce region_create() functions).

TEST=Able to build google/rex.

Change-Id: I0dbd839e3573d5c74375911903c8f9d6a66bbf28
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/83886
Reviewed-by: Jakub Czapiga <czapiga@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/commonlib/include/commonlib/region.h

@@ -9,6 +9,7 @@
 #include <commonlib/bsd/cb_err.h>
 #include <commonlib/bsd/helpers.h>
 #include <commonlib/mem_pool.h>
+#include <stdint.h>
 
 /*
  * Region support.