profiling: Include unistd.h to avoid compiler warnings

profiling: Include unistd.h to avoid compiler warnings

RHEL8's gcc-8.5.0 emits these warnings:

src/profiling.c: In function ‘get_signalfd’:
src/profiling.c:32:3: warning: implicit declaration of function ‘close’; did you mean ‘pclose’? [-Wimplicit-function-declaration]
   close(fd);
   ^~~~~
   pclose
src/profiling.c: In function ‘check_signalfd’:
src/profiling.c:42:6: warning: implicit declaration of function ‘read’; did you mean ‘fread’? [-Wimplicit-function-declaration]
  if (read(fd, &info, sizeof(info)) < (signed)sizeof(info))
      ^~~~
      fread

Fixes: 868040f892238 ("configure: Implement --enable-profiling option")
Signed-off-by: Phil Sutter <phil@nwl.cc>