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/acpi/soundwire.c

@@ -7,7 +7,6 @@
 #include <device/soundwire.h>
 #include <stdbool.h>
 #include <stdint.h>
-#include <string.h>
 
 /* Specification-defined prefix for SoundWire properties. */
 #define SDW_PFX "mipi-sdw-"