perf tools: Fix unwind build on i386

perf tools: Fix unwind build on i386

EINVAL may not be defined when building unwind-libunwind.c with
REMOTE_UNWIND_LIBUNWIND, resulting in a compiler error in
LIBUNWIND__ARCH_REG_ID().  Its only caller, access_reg(), only checks
for a negative return value and doesn't care what it is.  So change
-EINVAL to -1.

Fixes: 52ffe0ff02fc ("Support x86(32-bit) cross platform callchain unwind.")
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Gbp-Pq: Topic bugfix/x86
Gbp-Pq: Name perf-tools-fix-unwind-build-on-i386.patch