build: fix extract.pl for cross compilation

extract.pl should invoke the C preprocessor for the target system, not the
host.

* vtysh/extract.pl.in: use @CPP@ to get target cpp

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/vtysh/extract.pl.in b/vtysh/extract.pl.in
index 2dbaf0a..3776304 100755
--- a/vtysh/extract.pl.in
+++ b/vtysh/extract.pl.in
@@ -63,7 +63,7 @@
 foreach (@ARGV) {
     $file = $_;
 
-    open (FH, "cpp -DHAVE_CONFIG_H -DVTYSH_EXTRACT_PL -DHAVE_IPV6 -I@top_builddir@ -I@srcdir@/ -I@srcdir@/.. -I@top_srcdir@/lib -I@top_builddir@/lib -I@top_srcdir@/isisd/topology @CPPFLAGS@ $file |");
+    open (FH, "@CPP@ -DHAVE_CONFIG_H -DVTYSH_EXTRACT_PL -DHAVE_IPV6 -I@top_builddir@ -I@srcdir@/ -I@srcdir@/.. -I@top_srcdir@/lib -I@top_builddir@/lib -I@top_srcdir@/isisd/topology @CPPFLAGS@ $file |");
     local $/; undef $/;
     $line = <FH>;
     close (FH);