tests: Add missing dependency on glib-2.0

It has previously been implicitly pulled in by libsysprof-capture, but
that will change in future.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Helps: #40

src/tests/test-mapped-ring-buffer.c

@@ -1,5 +1,6 @@
 #include "mapped-ring-buffer.h"
 
+#include <glib.h>
 #include <stdint.h>
 
 static gsize real_count;