fix compile error in recent master

[edap]

Without this fix, the addon does not compile with the latest OF master version (that soon will be the new release, 0.10), this was the error

/ofxMaxim/libs/maxiGrains.h:128: error: no return statement in function returning non-void [-Werror=return-type]
virtual double play() {}

[ClaudiaMirabile]

The correction doesn't work for me:
Error C2577 'maxiGrainBase': a destructor cannot have a return type (compiling source file src\ofApp.cpp) 10_MaxigranularSynthesys

[mzed]

Isn't the problem in the play() method, not the destructor?

[edap]

You are right @mzed, my bad. I will test it at home and change the commit.

[rychrd]

@edap I get the same error on RPi , but not Mac...Making play() method 'pure' virtual seems to fix it, but would be good if someone could double check i've not broken something else..

class maxiGrainBase {
public:
virtual double play() = 0;
virtual ~maxiGrainBase() {}
bool finished;
};