[gdb/build] Fix typo sys/sockets.h -> sys/socket.h

[gdb/build] Fix typo sys/sockets.h -> sys/socket.h

I'm running into a build breaker:
...
src/gdb/ser-tcp.c:65:13: error: conflicting declaration ‘typedef int
socklen_t’
   65 | typedef int socklen_t;
      |             ^~~~~~~~~
In file included from ../gnulib/import/unistd.h:40,
                 from
/home/vries/gdb_versions/devel/src/gdb/../gnulib/import/pathmax.h:42,
                 from
/home/vries/gdb_versions/devel/src/gdb/../gdbsupport/common-defs.h:120,
                 from src/gdb/defs.h:28,
                 from src/gdb/ser-tcp.c:20:
/usr/include/unistd.h:277:21: note: previous declaration as ‘typedef
__socklen_t socklen_t’
  277 | typedef __socklen_t socklen_t;
      |                     ^~~~~~~~~
...
after commit 05a6b8c28b "Don't unnecessarily redefine 'socklen_t' type in
MinGW builds".

The root cause is a typo in gdb/configure.ac, using sys/sockets.h where
sys/socket.h was meant:
...
AC_CHECK_HEADERS([sys/sockets.h])
...

Fix the typo.

Build and tested on x86_64-linux.

gdb/ChangeLog:

2020-07-27  Tom de Vries  <tdevries@suse.de>

	* configure.ac: Fix sys/sockets.h -> sys/socket.h typo.
	* config.in: Regenerate.
	* configure: Regenerate.