Fix load statements

ortools/math_opt/cpp/BUILD.bazel

@@ -11,6 +11,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
+
 # External users should depend only on ":math_opt" and include
 # "math_opt.h". Hence other libraries are private.
 package(default_visibility = [