[ELF] Fix a gcc -Wextra warning
[ELF] Fix a gcc -Wextra warning

Extracted from D61046.

warning: enumeral and non-enumeral type in conditional expression [-Wextra]

Cast SHF_ALLOC to avoid that.

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@359070 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld