[MISC] fix compiling error for MSVC. 'FALL_THROUGH': undeclared ident…

[zero-rp]

…ifier

[litespeedtech]

You patch maybe fix something for MSVC, but break everything else that does not support fallthrough attribute.
A proper fix need to combine

#  ifndef _MSC_VER
#    if defined __has_attribute && __has_attribute (fallthrough)

to one #if
Not sure why it is needed, __has_attribute (fallthrough) return true in MSVC, but does not support __attribute__ ((fallthrough))?

[zero-rp]

I will submit a new patch later,When I compiled using cmake in vs2022, the compiler reported the following:
.\lsquic\src\liblsquic\ls-qpack\lsqpack.c(57): error C2220: The following warnings are considered errors
.\lsquic\src\liblsquic\ls-qpack\lsqpack.c(57): warning C4067: Unexpected flag after preprocessor instruction - line break should be entered

[CL-Jeremy]

Hi, I just opened a new pull request #80 with alternative tests for fallthrough. Can you try if this works for your version of VS 2022 if you have time? Thanks in advance!