CMakeLists.txt: export include directories in install(TARGETS)

Without that, if olm is installed to a non-standard location, the user code might run CMake configuration fine but further building will fail.
Signed-off-by: Alexey Rusakov <Alexey.Rusakov@pm.me>

CMakeLists.txt

@@ -72,7 +72,9 @@ set(INSTALL_CONFIGDIR ${CMAKE_INSTALL_LIBDIR}/cmake/Olm)
 install(TARGETS olm
     EXPORT olm-targets
     LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
-    ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR})
+    ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
+    INCLUDES DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
+)
 
 # The exported target will be named Olm.
 set_target_properties(olm PROPERTIES EXPORT_NAME Olm)