gcov: add missing gcov.h includes

Fixes the following "implicit declaration" warning (13 occurrences in
total) when building with gcov support:

    $ pacman -Q gcc10
    gcc10 1:10.2.0-3
    $ CC=gcc-10 && export CC
    $ ./configure --prefix=/usr --enable-apparmor --enable-gcov >/dev/null
    $ make >/dev/null
    appimage.c: In function ‘appimage_set’:
    appimage.c:140:2: warning: implicit declaration of function ‘__gcov_flush’ [-Wimplicit-function-declaration]
      140 |  __gcov_flush();
          |  ^~~~~~~~~~~~
    interface.c: In function ‘print_sandbox’:
    interface.c:149:3: warning: implicit declaration of function ‘__gcov_flush’ [-Wimplicit-function-declaration]
      149 |   __gcov_flush();
          |   ^~~~~~~~~~~~
    netstats.c: In function ‘netstats’:
    netstats.c:246:4: warning: implicit declaration of function ‘__gcov_flush’ [-Wimplicit-function-declaration]
      246 |    __gcov_flush();
          |    ^~~~~~~~~~~~
    [...]

Note: The commands above were executed from makepkg, while building
firejail-git from the AUR.

Note2: gcc-10 was used because the build fails with the current gcc
version (11.1.0) on Artix Linux.  The failure happens because
__gcov_flush was removed on gcc 11.1.0[1]; this will be addressed later.

Note3: The following command helped find the affected files:

    $ git grep -Fl __gcov -- src

[1] https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=811b7636cb8c10f1a550a76242b5666c7ae36da2

1 files changed, 4 insertions, 0 deletions

diff --git a/src/firemon/netstats.c b/src/firemon/netstats.c
index 850959eb3..23d228e26 100644
--- a/src/firemon/netstats.c
+++ b/src/firemon/netstats.c
@@ -24,6 +24,10 @@
 #include <sys/stat.h>
 #include <unistd.h>
 
+#ifdef HAVE_GCOV
+#include <gcov.h>
+#endif
+
 #define MAXBUF 4096
 
 // ip -s link: device stats