Add src/raygui.c to .gitignore

...since this file is generated by Makefile while building with
'RAYLIB_MODULE_RAYGUI=TRUE'

.gitignore

@@ -113,3 +113,6 @@ docgen_tmp/
 
 # Parser stuff
 parser/raylib_parser
+
+# Raygui auto-generated source file
+src/raygui.c