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/core/select.c b/src/core/select.c
index 026d457..72f794f 100644
--- a/src/core/select.c
+++ b/src/core/select.c
@@ -36,7 +36,7 @@
 #include <osmocom/core/stat_item.h>
 #include <osmocom/core/stats_tcp.h>
 
-#include "../config.h"
+#include "config.h"
 
 #if defined(HAVE_SYS_SELECT_H) && defined(HAVE_POLL_H)
 #include <sys/select.h>