Don't include @libs@ in libvirt.pc.in file

The libvirt.pc file we install is ending up polluted with a
load of compiler flags that should be private to the libvirt
build. eg

Libs: -L${libdir} -lvirt -ldl -O2 -g -pipe -Wall \
      -Wp,-D_FORTIFY_SOURCE=2 -fexceptions \
      -fstack-protector-strong --param=ssp-buffer-size=4 \
      -grecord-gcc-switches  -m64 -mtune=generic

this is caused by including @libs@ in the Libs: line of the
pkgconfig.pc.in file.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

libvirt.pc.in

@@ -12,5 +12,5 @@ Name: libvirt
 Version: @VERSION@
 Description: libvirt library
 Requires:
-Libs: -L${libdir} -lvirt @LIBS@
+Libs: -L${libdir} -lvirt
 Cflags: -I${includedir}