Adds SQLCipher to the HEADER_SEARCH_PATHS of FMDB within the SQLCiphe…

[R4N]

…r subspec.

When installing FMDB/SQLCipher via cocoa pods and using Swift, the proper SQLCipher copy of sqlite3.h isn't located so the sqlite3_key and sqlite3_rekey functions aren't found.

This should hopefully resolve these issues:
#591
#618
#612

and accomplish the same thing as this PR (without changing the import statements/breaking direct integrations):
#594

A current workaround is to use a post_install hook in the Podfile:

post_install do |installer|
        installer.pods_project.targets.each do |target|
            if target.name == "FMDB"
               target.build_configurations.each do |config|
                   header_search = {"HEADER_SEARCH_PATHS" => "SQLCipher"}
                   config.build_settings.merge!(header_search)
               end
            end
        end
    end

[ccgus]

Crossing my fingers and hoping for the best here. I don't use Cocoa Pods, so…