egl: fix cast to silence compiler warning

egl: fix cast to silence compiler warning

eglcurrent.c: In function '_eglSetTSD':
eglcurrent.c:57:4: warning: passing argument 2 of 'tss_set' discards
'const' qualifier from pointer target type [enabled by default]
    tss_set(_egl_TSD, (const void *) t);
    ^
In file included from ../../../include/c11/threads.h:72:0,
                 from eglcurrent.c:32:
../../../include/c11/threads_posix.h:357:1: note: expected 'void *'
but argument is of type 'const void *'
 tss_set(tss_t key, void *val)
 ^

Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>