[Flang] Build fix without precompiled headers

The header semantics.h is added implitly in the precompiled headers, but
the build was failing when precompiled headers are disabled (e.g.
using CMAKE_DISABLE_PRECOMPILE_HEADERS=ON):

```
../_src/flang/lib/Semantics/canonicalize-omp.cpp: In constructor ‘Fortran::semantics::CanonicalizationOfOmp::CanonicalizationOfOmp(Fortran::semantics::SemanticsContext&)’:
../_src/flang/lib/Semantics/canonicalize-omp.cpp:31:38: error: invalid use of incomplete type ‘class Fortran::semantics::SemanticsContext’
   31 |       : context_{context}, messages_{context.messages()} {}
      |                                      ^~~~~~~
In file included from ../_src/flang/lib/Semantics/canonicalize-omp.cpp:9:
../_src/flang/lib/Semantics/canonicalize-omp.h:17:7: note: forward declaration of ‘class Fortran::semantics::SemanticsContext’
   17 | class SemanticsContext;
      |       ^~~~~~~~~~~~~~~~
compilation terminated due to -fmax-errors=1.
```

Author: Meinersbur

flang/lib/Semantics/canonicalize-omp.cpp

@@ -9,6 +9,7 @@
 #include "canonicalize-omp.h"
 #include "flang/Parser/parse-tree-visitor.h"
 #include "flang/Parser/parse-tree.h"
+#include "flang/Semantics/semantics.h"
 
 // After Loop Canonicalization, rewrite OpenMP parse tree to make OpenMP
 // Constructs more structured which provide explicit scopes for later