bconv2d_int8_DIDO kernel may not have proper respect for the -DNN_USE_REF flag

[keithm-xmos]

When compiling with the -DNN_USE_REF flag, the following linker errors occur:

lib_nn/lib_nn/src/asm/bconv2d_int8_DIDO_impl.S: Error: Multiple definition of 'bconv2d_int8_DIDO_impl'
bnn_conv2d_int8_out.c: Error: first defined here
lib_nn/lib_nn/src/asm/bconv2d_int8_DIDO_impl.S: Error: Multiple definition of 'bconv2d_int8_DIDO_impl.nstackwords'
bnn_conv2d_int8_out.c: Error: first defined here
lib_nn/lib_nn/src/asm/bconv2d_int8_DIDO_impl.S: Error: Multiple definition of 'bconv2d_int8_DIDO_impl.maxcores'
bnn_conv2d_int8_out.c: Error: first defined here
ib_nn/lib_nn/src/asm/bconv2d_int8_DIDO_impl.S: Error: Multiple definition of 'bconv2d_int8_DIDO_impl.maxtimers'
bnn_conv2d_int8_out.c: Error: first defined here
lib_nn/lib_nn/src/asm/bconv2d_int8_DIDO_impl.S: Error: Multiple definition of 'bconv2d_int8_DIDO_impl.maxchanends'
bnn_conv2d_int8_out.c: Error: first defined here