Fix for compiling on newer versions of gcc

Compiling on Ubuntu 12.04 I get the following:

    $ make
    gcc -lm -o heatmap heatmap.c
    /tmp/cc1kQt5n.o: In function `loglinear_buckets':
    heatmap.c:(.text+0x267): undefined reference to `pow'
    collect2: ld returned 1 exit status
    make: *** [heatmap] Error 1
    $ gcc --version
    gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3

This change will resolve the error, and appears to still work on Mac OS X 10.8, Solaris 11.1 and Debian 6, each of which has a gcc version older than 4.6.

My reference for this fix is this [Stack Overflow question](http://stackoverflow.com/questions/12644922/).