fix linker error (undefined reference to symbol 'pthread_create@@GLIBC_2...

Got this while compiling:
/usr/bin/ld: io/rtlsdrtuner.o: undefined reference to symbol 'pthread_create@@GLIBC_2.1'

Fixed by adding -lpthread to Makefile.am

System is: 
Linux Mint 17 MATE 32-bit
Linux 3.13.0-24-generic #47-Ubuntu SMP Fri May 2 23:31:42 UTC 2014 i686 

Also tested on Linux Mint 16 MATE 32-bit.