tests: Add SELINUX_LIBS to fix viridentitytest linker bug

tests: Add SELINUX_LIBS to fix viridentitytest linker bug

In a clean build system (Ubuntu 14.04), the viridentitytest failed to compile.
Even if all the SELINUX libraries and depedencies are installed. See the error
message below:

[...]
  CC       viridentitytest.o
  CCLD     viridentitytest
/usr/bin/ld: viridentitytest.o: undefined reference to symbol
                                                       'security_disable'
//lib/x86_64-linux-gnu/libselinux.so.1: error adding symbols: DSO missing
                                                           from command line
collect2: error: ld returned 1 exit status
make: *** [viridentitytest] Error 1

Simply adding the variable SELINUX_LIBS in viridentitytest rules of
Makefile.am to include SELINUX libraries into viridentitytest solved that
compilation issue.

Signed-off-by: Julio Faracco <jcfaracco@gmail.com>
Signed-off-by: Eric Blake <eblake@redhat.com>