arm/convert.c: Add missing convert_init implementation
arm/convert.c: Add missing convert_init implementation

osmo-trx.cpp calls convert_init, which in case of building using
--with-neon is not implemented and the compiler stops with an error.

Error was introduced in 7e07cf23464532ca156fe4ea4430928da2cbbff1.

Related: OS#2720

Change-Id: I9840d374d13b525b97f978ea0c5ed9e8421072a0

ttsou/osmo-trx,ttsou/osmo-trx,ttsou/osmo-trx,osmocom/osmo-trx,ttsou/osmo-trx,osmocom/osmo-trx,osmocom/osmo-trx,osmocom/osmo-trx