[acpi] Fix spurious uninitialised-variable warning on some gcc versions

Reported-by: Christian Nilsson <nikize@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/acpi.c

 static uint8_t acpi_checksum ( userptr_t table ) {
 	struct acpi_header acpi;
 	uint8_t sum = 0;
-	uint8_t data;
+	uint8_t data = 0;
 	unsigned int i;
 
 	/* Read table length */