Revert "Use BUILD_ASSERT_MSG instead of _Static_assert in Zephyr"

This reverts commit 1b8d7fb355f86284bcaaa3bb79da930ece83ca7c.
diff --git a/boot/bootutil/include/bootutil/image.h b/boot/bootutil/include/bootutil/image.h
index 6989049..b2d1d94 100644
--- a/boot/bootutil/include/bootutil/image.h
+++ b/boot/bootutil/include/bootutil/image.h
@@ -22,10 +22,6 @@
 
 #include <inttypes.h>
 
-#ifdef __ZEPHYR__
-#include <toolchain/gcc.h>
-#endif
-
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -109,13 +105,8 @@
 
 #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,