musl: fix hash of license file

COPYRIGHT file has been updated between version 1.1.20 and 1.1.21:
https://git.musl-libc.org/cgit/musl/commit/COPYRIGHT?id=c50985d5c8e316c5c464f352e79eeebfed1121a9

Fixes:
 - http://autobuild.buildroot.org/results/8cfa70b906221442c9e6dfd46b64011c987d24bf

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/musl/musl.hash

@@ -1,4 +1,4 @@
 # Locally calculated after checking pgp signature from
 # http://www.musl-libc.org/releases/musl-1.1.21.tar.gz.asc
 sha256  c742b66f6f49c9e5f52f64d8b79fecb5a0f6e0203fca176c70ca20f6be285f44  musl-1.1.21.tar.gz
-sha256  15d9afbf84041872b4d840ed7d165d3eee786ff3f97e703b10467c259ff4e7d9  COPYRIGHT
+sha256  a3ae1b9fc5d4938f5734734383b9813d27a5652df23010c6f9d4c5419b239a41  COPYRIGHT