build: fix VPATH 'make check'

build: fix VPATH 'make check'

A VPATH build 'make check' was failing with:

  GEN      check-driverimpls
Can't open ../../src/../../src/lxc/lxc_monitor_protocol.h: No such file or directory at ../../src/check-driverimpls.pl line 29, <> line 27153.
Can't open ../../src/../../src/lxc/lxc_monitor_protocol.c: No such file or directory at ../../src/check-driverimpls.pl line 29, <> line 27153.
...
  GEN      check-aclrules
cannot read ../../src/../../src/remote/remote_protocol.x at ../../src/check-aclrules.pl line 128.

because $(srcdir) was being prepended to file names that already
included it.

* src/Makefile.am (check-driverimpls): Don't add srcdir twice.

Signed-off-by: Eric Blake <eblake@redhat.com>