Add PGTYPESchar_free() to avoid cross-module problems on Windows.

On Windows, it is sometimes important for corresponding malloc() and
free() calls to be made from the same DLL, since some build options can
result in multiple allocators being active at the same time.  For that
reason we already provided PQfreemem().  This commit adds a similar
function for freeing string results allocated by the pgtypes library.

Author: Takayuki Tsunakawa
Reviewed-by: Kyotaro Horiguchi
Discussion: https://postgr.es/m/0A3221C70F24FB45833433255569204D1F8AD5D6%40G01JPEXMBYT05

src/interfaces/ecpg/test/expected/preproc-outofscope.c

@@ -28,6 +28,8 @@
 #ifndef PGTYPES_NUMERIC
 #define PGTYPES_NUMERIC
 
+#include <pgtypes.h>
+
 #define NUMERIC_POS						0x0000
 #define NUMERIC_NEG						0x4000
 #define NUMERIC_NAN						0xC000