gn: Switch to distro-neutral openharmony override

This enables the special OpenHarmony patches version of the gn tool for use with
any distro configuration which enables the "openharmony" in its DISTROOVERRIDES.

Signed-off-by: Esben Haabendal <esben.haabendal@huawei.com>

recipes-devtools/gn/gn_%.bbappend

@@ -6,4 +6,4 @@
 # GN function which was recently dropped, therefore downgrading GN to revision
 # compatible with OpenHarmony
 
-SRCREV:oniro-openharmony-linux = "5da62d5e9d0f10cb8ece7c30563a6a214c78b68d"
+SRCREV:openharmony = "5da62d5e9d0f10cb8ece7c30563a6a214c78b68d"