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 7e07cf2346.

Related: OS#2720

Change-Id: I9840d374d13b525b97f978ea0c5ed9e8421072a0

Transceiver52M/arm/convert.c

@@ -28,6 +28,9 @@
 void neon_convert_ps_si16_4n(short *, const float *, const float *, int);
 void neon_convert_si16_ps_4n(float *, const short *, int);
 
+void convert_init(void) {
+}
+
 /* 4*N 16-bit signed integer conversion with remainder */
 static void neon_convert_si16_ps(float *out,
 				 const short *in,