Make json-c include respect pkg-config --cflags

json-c.pc contains `Cflags: -I${includedir}/json-c`, so `<json-c/json.h>`
won't be found unless the parent directory is searched by default.

sway/commands.c

@@ -5,7 +5,7 @@
 #include <string.h>
 #include <strings.h>
 #include <stdio.h>
-#include <json-c/json.h>
+#include <json.h>
 #include "sway/commands.h"
 #include "sway/config.h"
 #include "sway/criteria.h"

sway/ipc-json.c

@@ -1,4 +1,4 @@
-#include <json-c/json.h>
+#include <json.h>
 #include <libevdev/libevdev.h>
 #include <stdio.h>
 #include <ctype.h>

sway/ipc-server.c

@@ -4,7 +4,7 @@
 #include <assert.h>
 #include <errno.h>
 #include <fcntl.h>
-#include <json-c/json.h>
+#include <json.h>
 #include <stdbool.h>
 #include <stdint.h>
 #include <stdio.h>