Fix assorted header files that failed to compile standalone.

We have a longstanding project convention that all .h files should
be includable with no prerequisites other than postgres.h.  This is
tested/relied-on by cpluspluscheck.  However, cpluspluscheck has not
historically been applied to most headers outside the src/include
tree, with the predictable consequence that some of them don't work.
Fix that, usually by adding missing #include dependencies.

The change in printf_hack.h might require some explanation: without
it, my C++ compiler whines that the function is unused.  There's
not so many call sites that "inline" is going to cost much, and
besides all the callers are in test code that we really don't care
about the size of.

There's no actual bugs being fixed here, so I see no need to back-patch.

Discussion: https://postgr.es/m/b517ec3918d645eb950505eac8dd434e@gaz-is.ru

src/interfaces/ecpg/ecpglib/ecpglib_extern.h

@@ -8,6 +8,7 @@
 #include "sqlda-native.h"
 #include "sqlda-compat.h"
 #include "ecpg_config.h"
+#include "ecpgtype.h"
 
 #ifndef CHAR_BIT
 #include <limits.h>

src/interfaces/ecpg/test/printf_hack.h

@@ -2,7 +2,7 @@
  * print_double(x) has the same effect as printf("%g", x), but is intended
  * to produce the same formatting across all platforms.
  */
-static void
+static inline void
 print_double(double x)
 {
 #ifdef WIN32

src/interfaces/libpq/fe-gssapi-common.h

@@ -16,8 +16,13 @@
 #include "libpq-fe.h"
 #include "libpq-int.h"
 
+#ifdef ENABLE_GSS
+
 void		pg_GSS_error(const char *mprefix, PGconn *conn,
 						 OM_uint32 maj_stat, OM_uint32 min_stat);
 bool		pg_GSS_have_ccache(gss_cred_id_t *cred_out);
 int			pg_GSS_load_servicename(PGconn *conn);
+
+#endif
+
 #endif							/* FE_GSSAPI_COMMON_H */