python-sip: fix installation of sip.h header

With the current python-sip package, the sip.h header is installed in
$(STAGING_DIR)/usr/include/python3.4/, while Python headers are
expected to be installed in $(STAGING_DIR)/usr/include/python3.4m/
(notice the 'm' at the end).

Since with this, the directory name for the headers and libraries is
not the same, this patch introduces separate PYTHON_SIP_LIB_DIR and
PYTHON_SIP_INCLUDE_DIR variables, and use them for the --destdir and
--incdir options.

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