gitweb: Fix up bogus $stylesheet declarations

gitweb: Fix up bogus $stylesheet declarations

This seems to be a pre-++ residual declaration and it wasn't good for
anything at all besides flooding the webserver errorlog with "omg, our in
the same scope!!" warnings.

[jc: the patch was bogus by defining the variable which defeated a
 later test that checked it with "defined", which I fixed up.]

Signed-off-by: Petr Baudis <redacted>
Signed-off-by: Junio C Hamano <redacted>