Change header export settings in FMDB and FMDB-IOS targets

These changes cause the FMDB-IOS library target to export headers so projects that depend on FMDB-IOS can find those headers in a natural place under $(BUILT_PRODUCTS_DIR)/usr/local/include.

The second commit updates the header export paths of both FMDB and FMDB-IOS so that headers can be imported like:

#import <FMDB/FMDatabase.h>

instead of

#import <FMDatabase.h>