use user's shell instead of hardwired "/bin/sh" (android needs this)

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/include/archive.h b/include/archive.h
index 181c187..b139dc5 100644
--- a/include/archive.h
+++ b/include/archive.h
@@ -84,6 +84,7 @@
 # endif
 #if ENABLE_FEATURE_TAR_TO_COMMAND
 	char* tar__to_command;
+	const char* tar__to_command_shell;
 #endif
 # if ENABLE_FEATURE_TAR_SELINUX
 	char* tar__global_sctx;