[flang][OpenMP] Silence warning in openmp-parsers.cpp

flang/lib/Parser/openmp-parsers.cpp:1655:43: warning
: logical not is only applied to the left hand side of comparison [-Wlogical-not
-parentheses]
 1655 | TYPE_PARSER(!StandaloneDirectiveLookahead >=
      |                                           ^~

Author: kparzysz

flang/lib/Parser/openmp-parsers.cpp

@@ -1627,7 +1627,7 @@ static constexpr auto StandaloneDirectiveLookahead{//
     "TARGET UPDATE"_sptok || "TARGET_UPDATE"_sptok};
 
 // Directives enclosing structured-block
-TYPE_PARSER(!StandaloneDirectiveLookahead >=
+TYPE_PARSER((!StandaloneDirectiveLookahead) >=
     construct<OmpBlockDirective>(first(
         "MASKED" >> pure(llvm::omp::Directive::OMPD_masked),
         "MASTER" >> pure(llvm::omp::Directive::OMPD_master),