build: admin backend needs libcap
build: admin backend needs libcap

Make sure to check for libcap availability for building the
admin backend to avoid potentially running into:

gvfsbackendadmin.c:27:28: fatal error: sys/capability.h: No such file or directory
 #include <sys/capability.h>
                            ^
compilation terminated.

https://bugzilla.gnome.org/show_bug.cgi?id=769747

gicmo/gvfs,gicmo/gvfs,gicmo/gvfs,gicmo/gvfs