gir+vapi: fixed C include path.
The headers are installed in ${includedir}/libuhttpmock-0.0/uhttpmock, not ${includedir}/libuhttpmock-0.0/libuhttpmock. Without this change, Vala source files using uhttpmock will produce bad C includes.
The headers are installed in ${includedir}/libuhttpmock-0.0/uhttpmock, not ${includedir}/libuhttpmock-0.0/libuhttpmock. Without this change, Vala source files using uhttpmock will produce bad C includes.