Fix hello_pi compilation error

When running rebuild.sh on Raspbian with latest updates, the following error occurs:

    cc -o hello_teapot.bin -Wl,--whole-archive triangle.o video.o models.o -lilclient -L/opt/vc/lib/ -lGLESv2 -lEGL -lopenmaxil -lbcm_host -lvcos -lvchiq_arm -lpthread -lrt -L../libs/ilclient -L../libs/vgfont -Wl,--no-whole-archive -rdynamic
    /usr/bin/ld: models.o: undefined reference to symbol 'sqrtf@@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.include:19: recipe for target 'hello_teapot.bin' failed
    make: *** [hello_teapot.bin] Error 1

Adding libm to linked libraries fixes this.