gn: bump to latest git HEAD

Since it is already possible to use a string subscript to
get the value of a scope member (i.e. foo["bar"] being the
same as foo.bar, but [] allowing expression evaluation),
this ensures that `defined(foo["bar"])` works as `defined(foo.bar)`
as well.

gn/Makefile

@@ -9,9 +9,9 @@ PKG_RELEASE:=1
 
 PKG_SOURCE_PROTO:=git
 PKG_SOURCE_URL:=https://gn.googlesource.com/gn.git
-PKG_SOURCE_DATE:=2023-02-17
-PKG_SOURCE_VERSION:=b25a2f8c2d33f02082f0f258350f5e22c0973108
-PKG_MIRROR_HASH:=28dcd0c0fe6fe8c459096acd0ff9ff4a982284ca680ff6edc1222dc88dac0b23
+PKG_SOURCE_DATE:=2023-04-05
+PKG_SOURCE_VERSION:=28b7b6c507eb808567e3aea446cd259f7691fddc
+PKG_MIRROR_HASH:=5b112442eabea4b906cc409237fac13b8efb4f5acb25989b1883d5ce96deebd2
 
 PKG_LICENSE:=BSD 3-Clause
 PKG_LICENSE_FILES:=LICENSE