ArmPkg/DefaultExceptionHandlerLib: add missing UefiLib include

ArmPkg/DefaultExceptionHandlerLib: add missing UefiLib include

Commit 31f5388006fc ("ArmPkg/DefaultExceptionHandlerLib: use console
if available") added calls to AsciiPrint() to the default exception
handler code, but the ARM version did not include UefiLib.h yet
(even though the .INF declares it unconditionally), resulting in
build breakage. So add the missing include.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>