Fix Linker Error on GCC-8.3

Fix a linker error for gcc-8.3 which requires executables/libraries to
link against the C++ filesystem library explicitly. Presumably later
versions of the gcc toolchain do this automatically or don't require it
for some other reason, this is at least the case for gcc-8.4.

Signed-off-by: Jack Frankland <jack.frankland@arm.com>
Change-Id: I301a5db11a2440c73beecb860478bd54a8132847