Update Makefile

I had to add this line for the build to complete successfully.  Otherwise I got the following error:

````
gcc -Lbuild/`uname -m`/test/ -lfftw3 -lpulse\
	build/`uname -m`/impulse/impulse.o build/`uname -m`/test/test-impulse.o\
	-o build/`uname -m`/test/test-impulse
/usr/bin/ld: build/armv6l/impulse/impulse.o: undefined reference to symbol 'sqrt@@GLIBC_2.4'
//lib/arm-linux-gnueabihf/libm.so.6: error adding symbols: DSO missing from command line
collect2: ld returned 1 exit status
Makefile:22: recipe for target 'test' failed
make: *** [test] Error 1
````