Add libcore to the list of ignored prefixes

contrib/ChangeLog:

	* gcc-changelog/git_commit.py (ignored_prefixes): Add
	'libgrust/rustc-lib/core/'.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>

Author: powerboat9

contrib/gcc-changelog/git_commit.py

@@ -150,6 +150,7 @@
     'gcc/testsuite/go.test/test/',
     'libffi/',
     'libgo/',
+    'libgrust/rustc-lib/core/',
     'libphobos/libdruntime/',
     'libphobos/src/',
     'libsanitizer/',