Use BUILD_ASSERT_MSG instead of _Static_assert in Zephyr

Signed-off-by: Rajavardhan Gundi <rajavardhan.gundi@intel.com>
diff --git a/boot/bootutil/include/bootutil/image.h b/boot/bootutil/include/bootutil/image.h
index b2d1d94..6989049 100644
--- a/boot/bootutil/include/bootutil/image.h
+++ b/boot/bootutil/include/bootutil/image.h
@@ -22,6 +22,10 @@
 
 #include <inttypes.h>
 
+#ifdef __ZEPHYR__
+#include <toolchain/gcc.h>
+#endif
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -105,8 +109,13 @@
 
 #define IS_ENCRYPTED(hdr) ((hdr)->ih_flags & IMAGE_F_ENCRYPTED)
 
+#ifdef __ZEPHYR__
+BUILD_ASSERT_MSG(sizeof(struct image_header) == IMAGE_HEADER_SIZE,
+               "struct image_header not required size");
+#else
 _Static_assert(sizeof(struct image_header) == IMAGE_HEADER_SIZE,
                "struct image_header not required size");
+#endif
 
 int bootutil_img_validate(struct image_header *hdr,
                           const struct flash_area *fap,