[flang] Fix -Wunneeded-internal-declaration in DirectivesCommon.h (NFC)

/llvm-project/flang/lib/Lower/DirectivesCommon.h:910:1:
error: 'static' function 'peelOuterConvert' declared in header file should be declared 'static inline' [-Werror,-Wunneeded-internal-declaration]
peelOuterConvert(Fortran::semantics::SomeExpr &expr) {
^
1 error generated.

flang/lib/Lower/DirectivesCommon.h

@@ -906,7 +906,7 @@ struct PeelConvert {
   }
 };
 
-static Fortran::semantics::SomeExpr
+static inline Fortran::semantics::SomeExpr
 peelOuterConvert(Fortran::semantics::SomeExpr &expr) {
   if (auto peeled = PeelConvert::visit(expr))
     return *peeled;