Add nolint for implicit borrowed_fd constructors

bpfloader uses both clang-tidy and this header, so the below lint
warnings are generated:

/work/aosp/system/core/base/include/android-base/unique_fd.h:261:18:
  warning: single-argument constructors must be marked explicit to
  avoid unintentional implicit conversions
  [google-explicit-constructor]
  /* implicit */ borrowed_fd(int fd) : fd_(fd) {}
                 ^
                 explicit

/work/aosp/system/core/base/include/android-base/unique_fd.h:263:18:
  warning: single-argument constructors must be marked explicit to
  avoid unintentional implicit conversions
  [google-explicit-constructor]
  /* implicit */ borrowed_fd(const unique_fd_impl<T>& ufd) : fd_(ufd.get()) {}
                 ^
                 explicit

Add NOLINT to quiet them.

Test: build without this lint warning
Change-Id: I5241938c33070b0fa39888289b8ca67d6d94ac73