string-helpers.h: Fix usage on musl libc

On musl, int32_t is defined in stdint.h.

Signed-off-by: Kylie McClain <kylie@somasis.com>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>

shared/string-helpers.h

@@ -28,6 +28,7 @@
 
 #include <stdbool.h>
 #include <stdlib.h>
+#include <stdint.h>
 #include <errno.h>
 #include <assert.h>