Add IWYU pragma to FIRDynamicLinks+FirstParty/FIRDynamicLinks (#6508)

Most FirstParty users will still need to use methods from FIRDynamicLinks.h
(shouldHandleDynamicLinkFromCustomSchemeURL:/dynamicLinkFromCustomSchemeURL: etc.).
Arguably, including both headers looks odd, hence add the IWYU pragma so that it's sufficient
to only use the FirstParty header.

Author: ahunt

FirebaseDynamicLinks/Sources/FIRDynamicLinks+FirstParty.h

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-#import "FirebaseDynamicLinks/Sources/Public/FirebaseDynamicLinks/FIRDynamicLinks.h"
+#import "FirebaseDynamicLinks/Sources/Public/FirebaseDynamicLinks/FIRDynamicLinks.h"  // IWYU pragma: export
 
 #import "FirebaseDynamicLinks/Sources/FIRDynamicLink+Private.h"