nwfilter: fix build error when pcap-config is not present

The compilation fails with the following error when pcap-config
is not present on the host:

nwfilter/nwfilter_learnipaddr.c:824:1: error: conflicting types for 'virNWFilterLearnIPAddress'
 virNWFilterLearnIPAddress(virNWFilterTechDriverPtr techdriver ATTRIBUTE_UNUSED,

 In file included from nwfilter/nwfilter_learnipaddr.c:57:0:
 nwfilter/nwfilter_learnipaddr.h:38:5: note: previous declaration of 'virNWFilterLearnIPAddress' was here
  int virNWFilterLearnIPAddress(virNWFilterTechDriverPtr techdriver,
Signed-off-by: NBrijesh Singh <brijesh.singh@amd.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>
Signed-off-by: NJán Tomko <jtomko@redhat.com>

src/nwfilter/nwfilter_learnipaddr.c

@@ -829,7 +829,7 @@ virNWFilterLearnIPAddress(virNWFilterTechDriverPtr techdriver ATTRIBUTE_UNUSED,
                           const char *filtername ATTRIBUTE_UNUSED,
                           virHashTablePtr filterparams ATTRIBUTE_UNUSED,
                           virNWFilterDriverStatePtr driver ATTRIBUTE_UNUSED,
-                          enum howDetect howDetect ATTRIBUTE_UNUSED)
+                          int howDetect ATTRIBUTE_UNUSED)
 {
     virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
                    _("IP parameter must be given since libvirt "