Move be-gssapi-common.h into src/include/libpq/

The file has been introduced in src/backend/libpq/ as of b0b39f72, but
all backend-side headers of libpq are located in src/include/libpq/.
Note that the identification path on top of the file referred to
src/include/libpq/ from the start.

Author: Michael Paquier
Reviewed-by: Stephen Frost
Discussion: https://postgr.es/m/20190607043415.GE1736@paquier.xyz

src/backend/libpq/auth.c

@@ -173,7 +173,7 @@ bool		pg_krb_caseins_users;
  *----------------------------------------------------------------
  */
 #ifdef ENABLE_GSS
-#include "be-gssapi-common.h"
+#include "libpq/be-gssapi-common.h"
 
 static int	pg_GSS_checkauth(Port *port);
 static int	pg_GSS_recvauth(Port *port);

src/backend/libpq/be-gssapi-common.c

@@ -14,7 +14,7 @@
 
 #include "postgres.h"
 
-#include "be-gssapi-common.h"
+#include "libpq/be-gssapi-common.h"
 
 /*
  * Helper function for getting all strings of a GSSAPI error (of specified

src/backend/libpq/be-secure-gssapi.c

@@ -16,8 +16,8 @@
 
 #include <unistd.h>
 
-#include "be-gssapi-common.h"
 #include "libpq/auth.h"
+#include "libpq/be-gssapi-common.h"
 #include "libpq/libpq.h"
 #include "libpq/libpq-be.h"
 #include "libpq/pqformat.h"