package/tmux: fix build

package/tmux: fix build

The bump to the latest libevent version exposed a latent bug in tmux,
which tries to re-define a variable exposed by the C library.

This breaks with uClibc-ng which defines it const char*, while glibc and
musl define it as non-const.

Fixes:
    http://autobuild.buildroot.org/results/31d/31d37c95b40d297b90a6181ec79c42ee1e2e5090/
    http://autobuild.buildroot.org/results/ccc/ccc9d2e995ba1eaf19f6b325b0af5ebeb8066e7f/
    http://autobuild.buildroot.org/results/8a2/8a28d43bd0c079c1b49b61b0c6ef7de79eda0549/

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>