src/webp/*.h: don't forward declare enums in C++

Change-Id: I36d3765e94d2b5529b321c186ccee1744785c5b3
fixes:
 error: ISO C++ forbids forward references to 'enum' types
since:
 28d25c8 replace 'typedef struct {} X;" by "typedef struct X X; struct X {};"
3 files changed