build: Fix building against RapidJSON 1.1.0 as sys dep

The last release of RapidJSON, 1.1.0 from 2016 (see
Tencent/rapidjson#1006 I guess), spells the variable in all caps:
https://github.com/Tencent/rapidjson/blob/v1.1.0/RapidJSONConfig.cmake.in
Use both spellings to accommodate both people on the last release and
people who have picked a newer git commit.

ext_libs/ext_libs.cmake

@@ -38,7 +38,7 @@ if(RAPIDJSON_SYS_DEP)
   # Since EXACT is not specified, any version compatible with 1.1.0 is accepted (>= 1.1.0)
   find_package(RapidJSON 1.1.0 CONFIG REQUIRED)
   add_library(RapidJSON INTERFACE)
-  target_include_directories(RapidJSON INTERFACE ${RapidJSON_INCLUDE_DIRS})
+  target_include_directories(RapidJSON INTERFACE ${RapidJSON_INCLUDE_DIRS} ${RAPIDJSON_INCLUDE_DIRS})
 else()
   add_library(RapidJSON INTERFACE)
   target_include_directories(RapidJSON SYSTEM INTERFACE "${CMAKE_CURRENT_LIST_DIR}/rapidjson/include")
@@ -127,4 +127,4 @@ if(VW_FEAT_CB_GRAPH_FEEDBACK)
   target_include_directories(mlpack_ensmallen SYSTEM INTERFACE ${CMAKE_CURRENT_LIST_DIR}/armadillo-code/include)
 
   target_include_directories(mlpack_ensmallen SYSTEM INTERFACE ${CMAKE_CURRENT_LIST_DIR}/ensmallen/include)
-endif()
+endif()