configure: set -Wextra-semi-stmt for clang with --enable-debug

To have it properly complain on empty statements with no effect.

Ref: #6376
Closes #6378

m4/curl-compilers.m4

@@ -948,6 +948,7 @@ AC_DEFUN([CURL_SET_COMPILER_WARNING_OPTS], [
           dnl clang 7 or later
           if test "$compiler_num" -ge "700"; then
             CURL_ADD_COMPILER_WARNINGS([tmp_CFLAGS], [assign-enum])
+            CURL_ADD_COMPILER_WARNINGS([tmp_CFLAGS], [extra-semi-stmt])
           fi
         fi
         ;;