[flang] Added missing _Complex for COMPLEX(10) C reduction API.

I noticed _Complex was missing for the long double case.
I might be missing something, but it looks like the compiler
does not call the C wrappers directly. Do we need to keep
the complex-reduction.[ch] at all?

Differential Revision: https://reviews.llvm.org/D148479

flang/runtime/complex-reduction.h

@@ -27,7 +27,7 @@ typedef _Lcomplex long_double_Complex_t;
 #else
 typedef float _Complex float_Complex_t;
 typedef double _Complex double_Complex_t;
-typedef long double long_double_Complex_t;
+typedef long double _Complex long_double_Complex_t;
 #endif
 
 #define REDUCTION_ARGS \