lib: bootm: add missing include

since the vxworks weaks are reimplement make
sure their prototypes are visible.

Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>

arch/arm/lib/bootm.c

@@ -15,6 +15,7 @@
 #include <common.h>
 #include <command.h>
 #include <image.h>
+#include <vxworks.h>
 #include <u-boot/zlib.h>
 #include <asm/byteorder.h>
 #include <libfdt.h>