[perl #112820] t/op/sprintf.t failure on FreeBSD 4.6

[perl #112820] t/op/sprintf.t failure on FreeBSD 4.6

t/op/sprintf.t fails one test on FreeBSD 4.6
*Everything* else passes. This is a pleasant surprise.

Patch adds a skip for FreeBSD 4, and should not skip on FreeBSD 5 and later.
Tested on FreeBSD 7 too. Due to the way versions don't map to real numbers,
4.6 > 4.11, whereas 4.6 <= 4.9 (and 4.9 <= 4.9, 4.10 <= 4.9 and 4.11 <= 4.9)
so the skip skips for all releases of FreeBSD 4.