WAF: Fix detection os sysname ...

WAF: Fix detection os sysname ...

Detection of sysname failed with stricter CFLAGS
"-Werrorr=implicit-function-declaration -Werror=implicit-int"

  Checking uname sysname type              : not found
  Checking uname machine type              : not found
  Checking uname release type              : not found
  Checking uname version type              : not found

../test.c: In function ‘main’:
../test.c:8:32: error: implicit declaration of function ‘printf’
    [-Werror=implicit-function-declaration]
                                printf("%s", n.sysname);
                                ^~~~~~
../test.c:8:32: warning: incompatible implicit declaration
    of built-in function ‘printf’
../test.c:8:32: note: include ‘<stdio.h>’ or provide a declaration of ‘printf’

Signed-off-by: Lukas Slebodnik <lslebodn@redhat.com>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>