Make heatshrink compile with std=c99 and using getopt().

The current target 'make heatshrink' results in "warning: implicit declaration of function ‘getopt’" and undeclared variable errors.
The -std=c99 prevents <unistd.h> from including <getopt.h> which makes the compile fail.

Including <getopt.h> additionally resolves this issue (as -std=gnu99 or -D_GNU_SOURCE is not a nice option).