configure: Fix spice probe

Non-existent $pkgconfig instead of $pkg_config was used when configure
probes for spice availability.
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>

configure

@@ -2207,9 +2207,9 @@ if test "$spice" != "no" ; then
 #include <spice.h>
 int main(void) { spice_server_new(); return 0; }
 EOF
-  spice_cflags=$($pkgconfig --cflags spice-protocol spice-server 2>/dev/null)
-  spice_libs=$($pkgconfig --libs spice-protocol spice-server 2>/dev/null)
-  if $pkgconfig --atleast-version=0.5.3 spice-server >/dev/null 2>&1 && \
+  spice_cflags=$($pkg_config --cflags spice-protocol spice-server 2>/dev/null)
+  spice_libs=$($pkg_config --libs spice-protocol spice-server 2>/dev/null)
+  if $pkg_config --atleast-version=0.5.3 spice-server >/dev/null 2>&1 && \
      compile_prog "$spice_cflags" "$spice_libs" ; then
     spice="yes"
     libs_softmmu="$libs_softmmu $spice_libs"