Use lowercase for shlwapi.lib rpcrt4.lib (facebook#6076)

Summary:
This fixes MinGW cross compilation from case-sensative file systems, at no harm to MinGW builds on  Windows.
Pull Request resolved: facebook#6076

Differential Revision: D18710554

fbshipit-source-id: a9f299ac3aa019f7dbc07ed0c4a79e19cf99b488

CMakeLists.txt

@@ -777,7 +777,7 @@ if(WITH_LIBRADOS)
 endif()
 
 if(WIN32)
-  set(SYSTEM_LIBS ${SYSTEM_LIBS} Shlwapi.lib Rpcrt4.lib)
+  set(SYSTEM_LIBS ${SYSTEM_LIBS} shlwapi.lib rpcrt4.lib)
   set(LIBS ${ROCKSDB_STATIC_LIB} ${THIRDPARTY_LIBS} ${SYSTEM_LIBS})
 else()
   set(SYSTEM_LIBS ${CMAKE_THREAD_LIBS_INIT})