Fix all references to config.h

config.h is created in $(top_buildir)/config.h.
Let's make sure all CPPFLAGS add correct -Ipath includes,
and that all code includes the correct file.

Change-Id: Ie9ea38bb009bc715b01cde4d66d181f7bec2e7bd
diff --git a/src/gsm/gsm_utils.c b/src/gsm/gsm_utils.c
index bbc7dd6..48341ae 100644
--- a/src/gsm/gsm_utils.c
+++ b/src/gsm/gsm_utils.c
@@ -92,7 +92,7 @@
 #include <time.h>
 #include <unistd.h>
 
-#include "../../config.h"
+#include "config.h"
 
 #if (!EMBEDDED)
 /* FIXME: this can be removed once we bump glibc requirements to 2.25: */