Fixed compile error on Mac: default arguments for lambda expressions

Summary:
Fixed the following compile error on Mac.

db/db_test.cc:8618:52: error: C++11 forbids default arguments for lambda expressions [-Werror,-Wlambda-extensions]
  auto gen_l0_kb = [this](int start, int size, int stride = 1) {
                                                   ^        ~
1 error generated.

Test Plan:
db_test

db/db_test.cc

@@ -8615,7 +8615,7 @@ TEST(DBTest, DynamicCompactionOptions) {
   options.max_bytes_for_level_multiplier = 4;
   DestroyAndReopen(&options);
 
-  auto gen_l0_kb = [this](int start, int size, int stride = 1) {
+  auto gen_l0_kb = [this](int start, int size, int stride) {
     Random rnd(301);
     std::vector<std::string> values;
     for (int i = 0; i < size; i++) {
@@ -8627,11 +8627,11 @@ TEST(DBTest, DynamicCompactionOptions) {
 
   // Write 3 files that have the same key range, trigger compaction and
   // result in one L1 file
-  gen_l0_kb(0, 128);
+  gen_l0_kb(0, 128, 1);
   ASSERT_EQ(NumTableFilesAtLevel(0), 1);
-  gen_l0_kb(0, 128);
+  gen_l0_kb(0, 128, 1);
   ASSERT_EQ(NumTableFilesAtLevel(0), 2);
-  gen_l0_kb(0, 128);
+  gen_l0_kb(0, 128, 1);
   dbfull()->TEST_WaitForCompact();
   ASSERT_EQ("0,1", FilesPerLevel());
   std::vector<LiveFileMetaData> metadata;
@@ -8646,9 +8646,9 @@ TEST(DBTest, DynamicCompactionOptions) {
     {"target_file_size_base", "65536"}
   }));
 
-  gen_l0_kb(0, 128);
+  gen_l0_kb(0, 128, 1);
   ASSERT_EQ("1,1", FilesPerLevel());
-  gen_l0_kb(0, 128);
+  gen_l0_kb(0, 128, 1);
   dbfull()->TEST_WaitForCompact();
   ASSERT_EQ("0,2", FilesPerLevel());
   metadata.clear();