Add missing nanopb flag (#2042)

Author: paulb777

FirebaseFirestore.podspec

@@ -68,7 +68,7 @@ Google Cloud Firestore is a NoSQL document database built for automatic scaling,
       # The nanopb pod sets these defs, so we must too. (We *do* require 16bit
       # (or larger) fields, so we'd have to set at least PB_FIELD_16BIT
       # anyways.)
-      'PB_FIELD_32BIT=1 PB_NO_PACKED_STRUCTS=1',
+      'PB_FIELD_32BIT=1 PB_NO_PACKED_STRUCTS=1 PB_ENABLE_MALLOC=1',
     'HEADER_SEARCH_PATHS' =>
       '"${PODS_TARGET_SRCROOT}" ' +
       '"${PODS_TARGET_SRCROOT}/Firestore/third_party/abseil-cpp" ' +