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>
diff --git a/shared/string-helpers.h b/shared/string-helpers.h
index 5dc75d6..c8ce449 100644
--- a/shared/string-helpers.h
+++ b/shared/string-helpers.h
@@ -28,6 +28,7 @@
 
 #include <stdbool.h>
 #include <stdlib.h>
+#include <stdint.h>
 #include <errno.h>
 #include <assert.h>