bpo-41302: Fix build with system libmpdec (GH-21481)

Move definition of UNUSED from modified headers of libmpdec to
_decimal.c itself. This makes the vendored source closer to the
standalone library and fixes build with --with-system-libmpdec.

Tested to build fine with either system libmpdec or the vendored one.

Modules/_decimal/_decimal.c

@@ -56,6 +56,11 @@
 
 #define BOUNDS_CHECK(x, MIN, MAX) x = (x < MIN || MAX < x) ? MAX : x
 
+#if defined(__GNUC__) && !defined(__INTEL_COMPILER)
+  #define UNUSED __attribute__((unused))
+#else
+  #define UNUSED
+#endif
 
 /* _Py_DEC_MINALLOC >= MPD_MINALLOC */
 #define _Py_DEC_MINALLOC 4

Modules/_decimal/libmpdec/mpdecimal.h

@@ -61,12 +61,6 @@ extern "C" {
   #define MPD_HIDE_SYMBOLS_END
 #endif
 
-#if defined(__GNUC__) && !defined(__INTEL_COMPILER)
-  #define UNUSED __attribute__((unused))
-#else
-  #define UNUSED
-#endif
-
 #if defined(_MSC_VER)
   #include "vccompat.h"
   #define EXTINLINE extern inline