ExportImageToMemory uses wrong return type

[danilwhale]

Before submitting a new issue, please verify and check:

• The issue is specific to Raylib-cs and not raylib
• I checked there is no similar issue already reported
• My code has no errors or misuse of Raylib-cs

Issue description

Original declaration looks like:

RLAPI unsigned char *ExportImageToMemory(Image image, const char *fileType, int *fileSize);

where unsigned char * should be byte* and not char*, because C char is equal to C# sbyte and unsigned char is equal to byte
https://github.com/ChrisDill/Raylib-cs/blob/502a583e076cf1595a12def81206be7f804002e5/Raylib-cs/interop/Raylib.cs#L1371

[chrisdill]

Thanks, fixed!