gdbserver/Makefile.in: add missing `-x c++`

When building with Clang, I get:

      CXX    nat/x86-linux-tdesc-ipa.o
    clang++: error: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Werror,-Wdeprecated]

Fix that by adding the missing `-x c++` in the rule building
`gdb/nat/*.c` files for the in-process agent.

Change-Id: Ie53e4b9a8b57bef9669397fdfaf21617107c7180
Approved-By: Tom Tromey <tom@tromey.com>