libeXosip: add dependency on host-pkg-config and remove useless flags

libeXosip: add dependency on host-pkg-config and remove useless flags

When libeXosip fails to use pkg-config to find libosip, it defaults to
thinking that libosip is installed in $(prefix)/lib and
$(prefix)/include, which is of course wrong. There was an attempt to
fix this by passing OSIP_CFLAGS and OSIP_LIBS variables to libeXosip
./configure script, but it still does not work:

checking pkg-config is at least version 0.9.0... ./configure: line 21035: /home/test/outputs/test-41/host/usr/bin/pkg-config: No such file or directory
no
checking for OSIP... configure: WARNING: assuming osip can be found in -I${prefix}/include and -L${exec_prefix}/lib

Therefore, the correct fix is to depend on host-pkg-config.

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