Fix vswitch-idl.ovsidl build rule with separate source and build dirs.

Fix vswitch-idl.ovsidl build rule with separate source and build dirs.

Before commit e3a880272 (lib: Specify idl location more precisely.), the
files named in VSWITCH_IDL_FILES were relative to the source directory.
That commit made them inconsistent: one remained relative to the source
directory, the other became relative to the build directory.  This meant
that if the source and build directories differed, the ovsdb-idlc
invocation had no change of succeeding.

This commit fixes the problem by making the file names consistently
relative to the build directory and then adjusting the ovsdb-idlc
invocation to expect that.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>