Fix build error causes by "unify version_string"

Commit 09c2e90c "unify version_string" introduced a build error,
fix it.

Signed-off-by: Andreas Biemann <andreas.devel@googlemail.com>
Signed-off-by: Wolfgang Denk <wd@denx.de>

include/version.h

@@ -37,6 +37,7 @@
 #define U_BOOT_VERSION_STRING U_BOOT_VERSION " (" U_BOOT_DATE " - " \
 	U_BOOT_TIME ")" CONFIG_IDENT_STRING
 
+#ifndef __ASSEMBLY__
 extern const char version_string[];
-
+#endif	/* __ASSEMBLY__ */
 #endif	/* __VERSION_H__ */