fontforge: fix compilation without 10.7 SDK

FontForge uses the FlatCarbon headers, which are gone in Mountain
Lion. To fix this, the formula was adjusted to use the 10.7 SDK.
However, this prevents compiling FontForge on a system without the
10.7 SDK. Fix this by replacing the FlatCarbon imports with the
correct imports to the non-flat headers.

Closes #17909.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>