result.h - fix bugprone-suspicious-semicolon warning

Fixes:
  system/core/base/include/android-base/result.h:
  133:94: warning: potentially unintended semicolon [bugprone-suspicious-semicolon]

Bernie says:
  it probably means that there's a parser bug with "if constexpr"

  maybe, at static analysis pass, the "if constexpr" was evaluated to false,
  and the compiler removed the "then" block from the AST...
  ... and then it thought you had written it that way :-)
  https://reviews.llvm.org/D46027



Test: builds
Bug: 153035880
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I25df8eeca4ec06b3180c1cd21b554fc583c5581a