gcov: fix gcov functions always declared as dummy

[kmk3]

Currently, the check to enable gcov relies on a non-existent macro due
to a typo, which looks like it would cause the dummy/empty versions of
the gcov functions to always be declared (even with --enable-gcov),
instead of the real ones from gcov.h. This commit fixes the typo
(HAS_GCOV -> HAVE_GCOV). See configure.ac for the macro declaration.

This amends commit 5106b2e ("gcov: use no-op functions if not
enabled", 2021-06-20) / PR #4376.

Occurrences of each macro with this commit applied:

$ git grep -F HAVE_GCOV | wc -l
16
$ git grep -F HAS_GCOV | wc -l
0

[kmk3]

This amends commit 5106b2e ("gcov: use no-op functions if not enabled",
2021-06-20) / PR #4376.

The commit that introduces the typo is from over 8 months ago and the only
other gcov-related commit since then that I found is commit b8d282c ("gcov",
2022-01-17) by @smitsohu.

So it looks like either the typo wasn't an issue or that no one has tried using
gcov since #4376.

IIRC I tried testing gcov when working on #4373/#4376 but couldn't get it to
work.

And with that I wonder, does anyone actually test firejail with gcov? When was
the last time anyone tried doing that? It does not seem to be hooked up in
tests or ci, so I assume that it has to be done manually.

[smitsohu]

@kmk3 TBH I sent b8d282c just because I realized that I had broken a pattern.

IIRC I tried testing gcov when working on #4373 but couldn't get it to
work.

Same here. If someone knows how to hold it - any hints would be appreciated.

[netblue30]

merged, thanks!