configure: check headers in sysroot, not in host's system headers

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

diff -durN libseccomp-1.0.0.orig/configure libseccomp-1.0.0/configure
--- libseccomp-1.0.0.orig/configure	2012-07-27 22:35:05.000000000 +0200
+++ libseccomp-1.0.0/configure	2012-10-27 00:12:50.739196219 +0200
@@ -205,7 +205,8 @@
 #
 
 # system seccomp includes
-if [[ -r "/usr/include/linux/seccomp.h" ]]; then
+# ${SYSROOT} added by buildroot for cross-compilation
+if [[ -r "${SYSROOT}/usr/include/linux/seccomp.h" ]]; then
 	opt_sysinc_seccomp="yes"
 else
 	opt_sysinc_seccomp="no"