syscall_trace_enter() and syscall_trace_leave() are only called from
within asm code and do not need to be declared in the .c at all.
Removing their reference fixes the build issue that was happening
with gcc 4.7.

Both Sven-Haegar Koch and Christoph Biedl confirmed this patch
addresses their respective build issues.

Cc: Sven-Haegar Koch <haegar@sdinet.de>
Cc: Christoph Biedl <linux-kernel.bfrz@manchmal.in-ulm.de>
Signed-off-by: Willy Tarreau <w@1wt.eu>