Removed leading underscore in dlsym-call

Before this change, glGenBuffers would raise "Function glGenBuffers is not available on this system" for me on OS X 10.7.3.

The manual for dlsym says:

> The symbol name passed to dlsym() is the name used in C source code.  For example to find the address of function foo(), you would pass "foo" as the symbol name. This is unlike the older dyld APIs which required a leading underscore.  If you looking up a C++ symbol, you need to use the mangled C++ symbol name.`