scripts/gdb: fix interrupts display after MCP on x86

The text line would not be appended to as it should have, it should have
been a '+=' but ended up being a '==', fix that.

Link: https://lkml.kernel.org/r/20250623164153.746359-1-florian.fainelli@broadcom.com


Fixes: b0969d76 ("scripts/gdb: print interrupts")
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>