src: Remove unused 'include <string.h>'

Found using:
diff <(git grep -l '#include <string.h>' -- src/) <(git grep -l 'STRINGIFY\|memcpy\|memmove\|memset\|memcmp\|memchr\|strdup\|strconcat\|strnlen\|strlen\|strchr\|strncpy\|strcpy\|strcmp\|strncmp\|strspn\|strcspn\|strtok_r\|strtok\|atol\|strrchr\|skip_atoi\|snprintf' -- src/) |grep -v vendorcode |grep '<'

Change-Id: I12802d0a6254b2fa39d59f485008bb2012f7b32d
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41913
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/southbridge/intel/i82371eb/fadt.c

@@ -4,7 +4,6 @@
  * Based on src/southbridge/via/vt8237r/vt8237_fadt.c
  */
 
-#include <string.h>
 #include <acpi/acpi.h>
 #include <device/device.h>
 #include <device/pci.h>