fix cflags when build with CASS_INSTALL_HEADER_IN_SUBDIR on

Author: remicollet

packaging/cassandra.pc.in

@@ -7,5 +7,5 @@ Name: cassandra
 Description: A C/C++ client driver for Apache Cassandra and DataStax Products
 Version: @version@
 Libs: -L${libdir} -l@PROJECT_LIB_NAME@
-Cflags:
+Cflags: -I${includedir}
 URL: https://github.com/datastax/cpp-driver/