cmd/exception: missing include string.h

When building qemu_arm64_defconfig with CMD_EXCEPTION a build error occurs:

    In file included from cmd/arm/exception64.c:87:
    include/exception.h: In function ‘exception_complete’:
    include/exception.h:41:23: error: implicit declaration of
    function ‘strlen’ [-Wimplicit-function-declaration]
       41 |                 len = strlen(argv[1]);
          |                       ^~~~~~

Add the missing include.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>

include/exception.h

@@ -6,6 +6,7 @@
  */
 
 #include <command.h>
+#include <string.h>
 
 static int do_exception(struct cmd_tbl *cmdtp, int flag, int argc,
 			char *const argv[])