Add plugin header install in CMakeLists.txt (#10025)

Summary:
Fixes https://github.com/facebook/rocksdb/issues/9987.
- Plugin specific headers can be specified by setting ${PLUGIN_NAME}_HEADERS in ${PLUGIN_NAME}.mk in the plugin directory.
- This is supported by the Makefile based build, but was missing from CMakeLists.txt.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10025

Test Plan:
- Add a plugin with ${PLUGIN_NAME}_HEADERS set in both ${PLUGIN_NAME}.mk and CMakeLists.txt
- Run Makefile based install and CMake based install and verify installed headers match

Reviewed By: riversand963

Differential Revision: D36584908

Pulled By: ajkr

fbshipit-source-id: 5ea0137205ccbf0d36faacf45d712c5604065bb5

CMakeLists.txt

@@ -1129,6 +1129,12 @@ if(NOT WIN32 OR ROCKSDB_INSTALL_ON_WINDOWS)
 
   install(DIRECTORY include/rocksdb COMPONENT devel DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}")
 
+  foreach (plugin ${PLUGINS})
+    foreach (header ${${plugin}_HEADERS})
+      install(FILES plugin/${plugin}/${header} DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/rocksdb/plugin/${plugin})
+    endforeach()
+  endforeach()
+
   install(DIRECTORY "${PROJECT_SOURCE_DIR}/cmake/modules" COMPONENT devel DESTINATION ${package_config_destination})
 
   install(