Added extern to fix test on ubuntu 22.04

[romanfbr]

gcc version 10.2.1:
Gives errors on make check (linkage)

/usr/bin/ld: codes.o:/src/turbofec/tests/codes.h:28: multiple definition of `conv_trunc'; conv_test.o:/src/turbofec/tests/codes.h:28: first defined here
/usr/bin/ld: codes.o:/src/turbofec/tests/codes.h:27: multiple definition of `lte_conv_pbch'; conv_test.o:/src/turbofec/tests/codes.h:27: first defined here
/usr/bin/ld: codes.o:/src/turbofec/tests/codes.h:26: multiple definition of `gmr1_conv_tch3_speech'; conv_test.o:/src/turbofec/tests/codes.h:26: first defined here
...................

Without this, make check does work on gcc 9.*

Also removed a deprecation warning from automake