[NFC] Silence compiler warning [-Wmissing-braces].

llvm/lib/Target/AArch64/AArch64SLSHardening.cpp:146:5: warning: suggest braces around initialization of subobject [-Wmissing-braces]
    "__llvm_slsblr_thunk_x0",  "__llvm_slsblr_thunk_x1",
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    {
llvm/lib/Target/AArch64/AArch64SLSHardening.cpp:168:5: warning: suggest braces around initialization of subobject [-Wmissing-braces]
    AArch64::X0,  AArch64::X1,  AArch64::X2,  AArch64::X3,  AArch64::X4,
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    {