tests: add missing include

The int32_t type is defined in stdint.h.

The musl C library is very conservative in the headers that it
internally includes, and stdint.h is not included by any other header,
unlike with glibc or uClibc, which breaks the build.

Add the missing header.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

tests/string-test.c

@@ -28,6 +28,7 @@
 #include <stdlib.h>
 #include <stdbool.h>
 #include <stdio.h>
+#include <stdint.h>
 #include <assert.h>
 
 #include "shared/string-helpers.h"