rpm: Add missing build requirement libffi-devel

The libffi files are needed to compile js on CentOS 7.
diff --git a/js/rpm/SPECS/js.spec b/js/rpm/SPECS/js.spec
index f7af3e2..5e0d1b1 100644
--- a/js/rpm/SPECS/js.spec
+++ b/js/rpm/SPECS/js.spec
@@ -40,6 +40,7 @@
 BuildRequires:	zip
 BuildRequires:	ncurses-devel
 BuildRequires:	autoconf213
+BuildRequires:	libffi-devel
 
 
 %description