http-backend: Let gcc check the format of more printf-type functions.

We already have these checks in many printf-type functions that have
prototypes which are in header files.  Add these same checks to
static functions in http-backend.c

Signed-off-by: Tarmigan Casebolt <tarmigan+git@gmail.com>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http-backend.c

@@ -108,6 +108,7 @@ static const char *get_parameter(const char *name)
 	return i ? i->util : NULL;
 }
 
+__attribute__((format (printf, 2, 3)))
 static void format_write(int fd, const char *fmt, ...)
 {
 	static char buffer[1024];
@@ -165,6 +166,7 @@ static void end_headers(void)
 	safe_write(1, "\r\n", 2);
 }
 
+__attribute__((format (printf, 1, 2)))
 static NORETURN void not_found(const char *err, ...)
 {
 	va_list params;
@@ -180,6 +182,7 @@ static NORETURN void not_found(const char *err, ...)
 	exit(0);
 }
 
+__attribute__((format (printf, 1, 2)))
 static NORETURN void forbidden(const char *err, ...)
 {
 	va_list params;