buffer: include stdarg header

This fixes the following warnings:

buffer.c: In function 'nft_buf_put':
buffer.c:53:2: warning: implicit declaration of function 'va_start' [-Wimplicit-function-declaration]
va_start(ap, fmt);
^
buffer.c:56:2: warning: implicit declaration of function 'va_end' [-Wimplicit-function-declaration]
va_end(ap);
^

---
 src/buffer.c | 1 +
 1 file changed, 1 insertion(+)

index 4fac110..d64f944 100644
@@ -9,6 +9,7 @@
 
 #include <stdio.h>
 #include <stdint.h>
+#include <stdarg.h>
 #include <inttypes.h>
 #include <string.h>
 #include <buffer.h>

1.9.1

Applied, thanks.