xstd: include missing headers for function prototypes

Fix the next warning:
> no previous prototype for ‘xstrcat’ [-Wmissing-prototypes]

1 files changed, 2 insertions, 0 deletions

diff --git a/src/xstd.c b/src/xstd.c
index f791ab2..f06f639 100644
--- a/src/xstd.c
+++ b/src/xstd.c
@@ -19,6 +19,8 @@
 #include <stdlib.h>
 #include <string.h>
 
+#include "xstd.h"
+
 char *xstrcat(const char *first, const char *second,
 	const char *delimiter)
 {