Makefile: enable -Werror=implicit-int and -Werror=strict-prototypes by default

The common error found in forward-ported/backported patches is missing
headers.  One recent example (files and function names are mangled):

    void foo(){}
    EXPORT_SYMBOL(foo);

gave only warning

    foo.c:12345678:5: warning: function declaration isn't a prototype [-Wstrict-prototypes]
    void foo(){}
         ^

    foo.c:12345679:5: warning: data definition has no type or storage class [enabled by default]
    EXPORT_SYMBOL(foo);
    foo.c:12345679:5: warning: type defaults to 'int' in declaration of 'EXORT_SYMBOL' [-Werror=implicit-int]

Now it's a fatal error. Tested on x86_64 allyesconfig.

[akpm@linux-foundation.org: fix typos in comments]
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>