meson build: fails to build stlinkv3 when usb is disabled

[MilhouseVH]

Presumably this is the correct fix, and it should have been added with 3eae695.

Without this PR, build v1.2 will fail when using the following meson options (cross-compiling for RPi):

-Dpciutils=false
-Dusb=false
-Dconfig_ft2232_spi=false
-Dconfig_usbblaster_spi=false

Error:

[12/82] Compiling C object 'flashrom@sha/stlinkv3_spi.c.o'.
FAILED: flashrom@sha/stlinkv3_spi.c.o 
/home/neil/projects/LibreELEC.tv/build.LibreELEC-RPi4.arm-9.80-devel/toolchain/bin/armv8a-libreelec-linux-gnueabihf-gcc -Iflashrom@sha -I. -I.. -fdiagnostics-color=always -pipe -D_FILE_OFFSET_BITS=64 -std=c99 -Wwrite-strings -Wno-unused-parameter -Wno-address-of-packed-member -Wno-missing-braces -D_DEFAULT_SOURCE -D_POSIX_C_SOURCE -D_BSD_SOURCE '-DFLASHROM_VERSION="v1.2"' -DHAVE_CLOCK_GETTIME=1 -DHAVE_STRNLEN=1 -DHAVE_UTSNAME=1 -march=armv8-a+crc -mtune=cortex-a53 -mabi=aapcs-linux -Wno-psabi -Wa,-mno-warn-deprecated -mfloat-abi=hard -mfpu=neon-fp-armv8 -Wall -O2 -fomit-frame-pointer -fPIC -DCONFIG_BUSPIRATE_SPI=1 -DCONFIG_DUMMY=1 -DCONFIG_LINUX_MTD=1 -DCONFIG_LINUX_SPI=1 -DCONFIG_PONY_SPI=1 -DCONFIG_SERPROG=1 -DCONFIG_STLINKV3_SPI=1 -DCONFIG_BITBANG_SPI=1 -MD -MQ 'flashrom@sha/stlinkv3_spi.c.o' -MF 'flashrom@sha/stlinkv3_spi.c.o.d' -o 'flashrom@sha/stlinkv3_spi.c.o' -c ../stlinkv3_spi.c
../stlinkv3_spi.c:30:10: fatal error: libusb.h: No such file or directory
   30 | #include <libusb.h>
      |          ^~~~~~~~~~
compilation terminated.

Full log: http://ix.io/2cgY