Update PULL_REQUEST_TEMPLATE.md (#18649)

Add C89 requirements

Author: gouchi

.github/PULL_REQUEST_TEMPLATE.md

@@ -3,6 +3,7 @@
 1. Rebase before opening a pull request
 2. If you are sending several unrelated fixes or features, use a branch and a separate pull request for each
 3. If possible try squashing everything in a single commit. This is particularly beneficial in the case of feature merges since it allows easy bisecting when a problem arises
+4. RetroArch codebase follows C89 coding rules for portability across many old platforms check using `C89_BUILD=1`
 
 ## Description