Add else clause to prevent some compiler warnings

Author: eddelbuettel

src/RcppArmadillo.cpp

@@ -111,5 +111,7 @@ int armadillo_get_number_of_omp_threads() {
 void armadillo_set_number_of_omp_threads(int n) {
 #ifdef _OPENMP
     omp_set_num_threads(n);
+#else
+    (void)(n);                  // prevent unused variable warning
 #endif
 }