perf: Set libunwind dir explicitly to point into staging sysroot

This helps it find the unwind library in sysroot always and not
look around and confuse with host

(From OE-Core rev: e914c03964c540493523b18660f8a4eaeec78e3d)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-kernel/perf/perf.bb

@@ -80,6 +80,7 @@ EXTRA_OEMAKE = '\
     NO_GTK2=1 \
     ${PACKAGECONFIG_CONFARGS} \
     TMPDIR="${B}" \
+    LIBUNWIND_DIR=${STAGING_EXECPREFIXDIR} \
 '
 
 EXTRA_OEMAKE += "\