Update default.xml

Bump mimalloc to v3.0.3

Author: compnerd

default.xml

@@ -63,7 +63,7 @@
 
   <project remote="github" name="madler/zlib" path="zlib" revision="refs/tags/v1.3.1" />
 
-  <project remote="github" name="microsoft/mimalloc" path="mimalloc" revision="refs/tags/v3.0.1" />
+  <project remote="github" name="microsoft/mimalloc" path="mimalloc" revision="refs/tags/v3.0.3" />
 
   <project remote="github" name="ninja-build/ninja" path="ninja" groups="notdefault,dependencies" revision="master" />
 </manifest>