[libc][NFC] Fix onre more -Wconversion warning in strtoul test code.

[mcgrathr]

Summary

The last change missed one spot.

libc/test/src/stdlib/strtoul_test.cpp

for (int first_digit = 0; first_digit <= 36; ++first_digit) {
                 errno = 0;
                 ASSERT_EQ(__llvm_libc::strtoul(small_string, nullptr, base),
                           0ul);
                 ASSERT_EQ(errno, 0);
               }
             } else {
               errno = 0;
               ASSERT_EQ(__llvm_libc::strtoul(small_string, nullptr, base),
-                        first_digit);
+                        static_cast<unsigned long int>(first_digit));
               ASSERT_EQ(errno, 0);
             }
           } else {
             errno = 0;
             ASSERT_EQ(__llvm_libc::strtoul(small_string, nullptr, base), 0ul);
             ASSERT_EQ(errno, 0);
           }
         }