CMakeLists,emscripten: use EXPORTED_RUNTIME_METHODS
CMakeLists,emscripten: use EXPORTED_RUNTIME_METHODS

rather than EXTRA_EXPORTED_RUNTIME_METHODS. this was deprecated in
2.0.18.

quiets a warning:
emcc: warning: EXTRA_EXPORTED_RUNTIME_METHODS is deprecated, please use
EXPORTED_RUNTIME_METHODS instead [-Wdeprecated]

https://emscripten.org/docs/introducing_emscripten/release_notes.html?highlight=exported_runtime_methods

Change-Id: I3acf22339a64f39a342051841a147744f6af954a

PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp