Add include directory for libxml on macOS

[sas]

Summary

Builds fail because libxml/xmlreader.h isn't found. Adding the include
directory to the include list fixes the issue. This is what we already do on
non-macOS platforms in the same file.

[lanza]

How did it ever find the file without the include_directories if it's at /usr/include/libxml2/libxml?

[sas]

I have no idea. I discovered this issue while working on our internal 7.0 branch.

lldb/trunk/cmake/modules/LLDBConfig.cmake

if (APPLE)
   add_definitions( -DLIBXML2_DEFINED )
   list(APPEND system_libs xml2
        ${CURSES_LIBRARIES}
        ${FOUNDATION_LIBRARY}
        ${CORE_FOUNDATION_LIBRARY}
        ${CORE_SERVICES_LIBRARY}
        ${SECURITY_LIBRARY}
        ${DEBUG_SYMBOLS_LIBRARY})
-
+  include_directories(${LIBXML2_INCLUDE_DIR})
 else()
   if (LIBXML2_FOUND)
     add_definitions( -DLIBXML2_DEFINED )
     list(APPEND system_libs ${LIBXML2_LIBRARIES})
     include_directories(${LIBXML2_INCLUDE_DIR})
   endif()
-
 endif()
 
 if( WIN32 AND NOT CYGWIN )
   set(PURE_WINDOWS 1)
 endif()
 
 if(NOT PURE_WINDOWS)
   set(CMAKE_THREAD_PREFER_PTHREAD TRUE)