Add a changelog notice for GetProcAddress change in 2.0.15 (#15675)

See #15674

Author: kripken

ChangeLog.md

@@ -404,6 +404,9 @@ See docs/process.md for more on how version tagging works.
 - Several pthreads exit-related fixes (#12985) (#10524).
 - Fix IDBFS syncing with existing directories (#13574).
 - Add libmodplug port and allow mod files to be played in SDL2 (#13478).
+- `emscripten_GetProcAddress` is now part of `libGL`. Normally the change is not
+  noticeable, unless you build in `STRICT` mode and do not already have `-lGL`
+  to link in that library. If not, add `-lGL`. (#13524)
 
 2.0.14: 02/14/2021
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