mesalib: Update version from 17.1.8 to 17.2.1

Release notes [1]:

> Mesa 17.2.1 Release Notes / September 17, 2017
>
> […]
>
> ### Bug fixes ###
>
>
> •   Bug 100613 - Regression in Mesa 17 on s390x (zSystems)
> •   Bug 101709 - [llvmpipe] piglit gl-1.0-scissor-offscreen regression
> •   Bug 102454 - glibc 2.26 doesn't provide anymore xlocale.h
> •   Bug 102467 - src/mesa/state_tracker/st_cb_readpixels.c:178]: (warning) Redundant assignment
> •   Bug 102502 - [bisected] Kodi crashes since commit 707d2e8b - gallium: fold u_trim_pipe_prim call from st/mesa to drivers
>
> […]

[1] https://www.mesa3d.org/relnotes/17.2.1.html

mesalib.be0

@@ -1,6 +1,6 @@
 #!/usr/bin/env beesh
 
-# BEE_VERSION mesalib-17.1.8-0
+# BEE_VERSION mesalib-17.2.1-0
 
 ## this file was created by bee init and should be executed to build a
 ## bee-package. (Additional hints are located at the end of this file.)