libffi: disable use of static exec trampolines

We encountered a blueman-applet segfault with libffi 3.4.2:
$ blueman-applet
blueman-applet 01.58.45 WARNING  PluginManager:148 __load_plugin: Not loading DhcpClient because its conflict has higher priority
blueman-applet 01.58.45 WARNING  TransferService:189 _make_share_path: Failed to get Download dir from XDG
blueman-applet 01.58.45 WARNING  PluginManager:148 __load_plugin: Not loading PPPSupport because its conflict has higher priority
Segmentation fault

This is because the static exec trampolines is enabled by default in
libffi 3.4.2. However it doesn't work with gobject-introspection[1].

[1] https://github.com/libffi/libffi/commit/132699b95d3ee4d889ea2a80540acf3300987dad



Signed-off-by: Yi Zhao <yi.zhao@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>