Added boost include dirs to pkg config file

Author: Alexander Damian

cppkafka.pc.in

@@ -11,4 +11,4 @@ Version: @CPPKAFKA_VERSION@
 Requires: librdkafka >= 0.9.4
 Requires.private:
 Libs: -L${libdir} -L${sharedlibdir} -L@RDKAFKA_ROOT_DIR@/lib -lcppkafka -lrdkafka -lpthread -lrt -lssl -lcrypto -ldl -lz
-Cflags: -I${includedir} -I${includedir}/cppkafka -I@RDKAFKA_ROOT_DIR@/include
+Cflags: -I${includedir} -I${includedir}/cppkafka -I@RDKAFKA_ROOT_DIR@/include -I@Boost_INCLUDE_DIRS@