Reference to FMDatabase.h from FMDatabaseAdditions.h

Fix an compilation error compiling using clang 3.3 on Fedora Linux

FMDatabaseAdditions.h:18:12: error: cannot find
interface declaration for 'FMDatabase'
@interface FMDatabase (FMDatabaseAdditions)
           ^
FMDatabaseAdditions.h:128:4: error: expected a type
- (FMResultSet*)getSchema;
   ^
FMDatabaseAdditions.h:152:4: error: expected a type
- (FMResultSet*)getTableSchema:(NSString*)tableName;
   ^