A few conversion warnings in the vst wrapper

General JUCE discussion
1

juce_VST_Wrapper.cpp:549:73: Implicit conversion changes signedness: ‘int’ to ‘const size_t’ (aka ‘const unsigned long’)
juce_VST_Wrapper.cpp:1608:62: Implicit conversion changes signedness: ‘unsigned long’ to ‘const int’
/juce_VST_Wrapper.cpp:1608:49: Implicit conversion changes signedness: ‘int’ to 'unsigned long’
juce_VST_Wrapper.cpp:1609:69: Implicit conversion changes signedness: ‘unsigned long’ to 'const int’
juce_VST_Wrapper.cpp:1609:71: Implicit conversion changes signedness: ‘const int’ to 'unsigned long’
juce_VST_Wrapper.cpp:1611:35: Implicit conversion changes signedness: ‘const int’ to ‘size_t’ (aka ‘unsigned long’)

2

Warnings fixed.