libmypaint: Update version from 1.3.0 to 1.6.1

GIMP 2.10.20 warns about using old versions, so update to the latest
one.

     WARNING: libmypaint lower than version 1.4.0 is known to crash when
              parsing MyPaint 2 brushes. Please update.

https://github.com/mypaint/libmypaint/releases

libmypaint.be0

@@ -1,6 +1,6 @@
 #!/usr/bin/env beesh
 
-# BEE_VERSION libmypaint-1.3.0-0
+# BEE_VERSION libmypaint-1.6.1-0
 
 SRCURL[0]="https://github.com/mypaint/libmypaint/releases/download/v${PKGVERSION}/libmypaint-${PKGVERSION}.tar.xz"