resample filter: Don't discard const.

Newer versions of libsamplerate made the data_in pointer const. This
causes the following warning:

	In file included from resample_filter.c:6:0:
	/usr/local/include/samplerate.h:177:6: note: expected 'float *' but argument is of type 'const float *'
	 void src_short_to_float_array (const short *in, float *out, int len) ;
	      ^
	resample_filter.c:173:7: warning: passing argument 1 of 'free' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
	In file included from para.h:11:0,
			 from resample_filter.c:10:
	/usr/include/stdlib.h:460:13: note: expected 'void *' but argument is of type 'const float *'
	 extern void free (void *__ptr) __THROW;

The problem is that we first convert the input from int16 to float
and use the data_in pointer as the target for the conversion.

Fix this by introducing a temporary non-const variable for the
converted input.