sim: pru: fix include ordering with sim-main.h

Make sure config.h is included before C library headers otherwise the
later libiberty.h include gets confused about asprintf state leading
to warnings like:
common/sim-utils.c:330:9:
	warning: implicit declaration of function 'vasprintf';
	did you mean 'xvasprintf'? [-Wimplicit-function-declaration]

sim/pru/ChangeLog

@@ -1,3 +1,7 @@
+2021-01-09  Mike Frysinger  <vapier@gentoo.org>
+
+	* sim-main.h: Include config.h.
+
 2021-01-08  Mike Frysinger  <vapier@gentoo.org>
 
 	* configure: Regenerate.

sim/pru/sim-main.h

@@ -19,6 +19,8 @@
 #ifndef PRU_SIM_MAIN
 #define PRU_SIM_MAIN
 
+#include "config.h"
+
 #include <stdint.h>
 #include <stddef.h>
 #include "pru.h"