Fix warnings reported by -Wpedantic

Previously CI hasn't been running with -Wpedantic (using autoconf), so
these haven't been reported previously.

- replace BUILD_ASSERT with static_assert()
- don't use %m GNU extension for printf
- don't use arithmetic on void*
- remove unused variables

Change-Id: I97d1acc908f06773b8b1ee95bfee80760fdc7a63
Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
diff --git a/utils/mctp-demux-daemon.c b/utils/mctp-demux-daemon.c
index fae4f35..7357d43 100644
--- a/utils/mctp-demux-daemon.c
+++ b/utils/mctp-demux-daemon.c
@@ -2,7 +2,9 @@
 
 #define _GNU_SOURCE
 
+#ifdef HAVE_CONFIG_H
 #include "config.h"
+#endif
 
 #define SD_LISTEN_FDS_START 3
 
@@ -64,7 +66,7 @@
 	struct binding *binding;
 	bool verbose;
 	int local_eid;
-	void *buf;
+	uint8_t *buf;
 	size_t buf_size;
 
 	int sock;