vsftpd: use pkg-config to get OpenSSL link flags

Instead of hardcoding -lssl -lcrypto as the linker flags to link with
OpenSSL, use pkg-config to fetch those flags. This allows to behave
properly when doing static linking, since pkg-config returns the
entire set of libraries we should link against.

Fixes:

  http://autobuild.buildroot.org/results/d71/d710ecfa9ed0a8e3f97d2010d3dabb372485c7f2/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Gergely Imreh <imrehg@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>