rpcbind: add missing dependency on host-pkgconf

rpcbind depends on libtirpc, which used to depend on
host-pkgconf. However, since commit
aee57627, the dependency of libtirpc
on host-pkgconf has been removed, because libtirpc no longer uses
PKG_CHECK_MODULES().

However, rpcbind does use PKG_CHECK_MODULES(). It used to work because
host-pkgconf was brought into the build by the dependency on libtirpc,
but that's no longer the case. Bottom line: we need to make rpcbind
depend on host-pkgconf, which is exactly what this commit does.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>