Fix missing corerun/ directory in make dist tarballs (mono/mono#17126)

```
 configure.ac:6695: error: required file 'netcore/corerun/Makefile.in' not found
 Makefile.am:24: error: required directory ./netcore/corerun does not exist
```

Commit migrated from mono/mono@d8040ba

src/mono/Makefile.am

@@ -28,7 +28,7 @@ SUBDIRS = $(build_with_msvc) mk po $(libgc_dir) llvm mono $(ikvm_native_dir) sup
 endif
 
 # Keep in sync with SUBDIRS
-DIST_SUBDIRS = $(build_with_msvc) m4 mk po $(libgc_dir) llvm mono ikvm-native support data runtime scripts man samples tools $(build_without_msvc) docs acceptance-tests netcore
+DIST_SUBDIRS = $(build_with_msvc) m4 mk po $(libgc_dir) llvm mono ikvm-native support data runtime scripts man samples tools $(build_without_msvc) docs acceptance-tests netcore netcore/corerun
 
 all: $(update_submodules)