Add modular headers support to Objective-C Firebase pods for static framework compatibility (#15506)

Co-authored-by: Nick Cooke <36927374+ncooke3@users.noreply.github.com>

FirebaseCore.podspec

@@ -56,6 +56,7 @@ Firebase Core includes FIRApp and FIROptions which provide central configuration
   s.dependency 'FirebaseCoreInternal', '~> 12.7.0'
 
   s.pod_target_xcconfig = {
+    'DEFINES_MODULE' => 'YES',
     'GCC_PREPROCESSOR_DEFINITIONS' => 'Firebase_VERSION=' + s.version.to_s,
     'HEADER_SEARCH_PATHS' => '"${PODS_TARGET_SRCROOT}"',
     'OTHER_CFLAGS' => '-fno-autolink'