nettle: fix the Segmentation fault

The commit[8ac8fa8ee1 nettle: update to 3.4.1]
add CFLAGS_append = " -std=c99" to silence the
below error for native build:
| ../nettle-3.4.1/rsa-sign-tr.c: In function 'sec_equal':
| ../nettle-3.4.1/rsa-sign-tr.c:243:3: error: 'for' loop initial declarations are only allowed in C99 mode
   for (size_t i = 0; i < limbs; i++)
   ^
| ../nettle-3.4.1/rsa-sign-tr.c:243:3: note: use option -std=c99 or -std=gnu99 to compile your code
| Makefile:263: recipe for target 'rsa-sign-tr.o' failed

But the above change will trigger below Segmentation
fault:
 # echo -n passwd| nettle-pbkdf2 -i 1 -l 16 salt
 [65534.886509] nettle-pbkdf2[708]: segfault at 1f594260 ip 00007f3332256998 sp 00007fff60d44410 error 4 in libnettle.so.6.5[7f3332244000+1d00]
 [65534.887525] Code: e8 6d db fe ff 44 01 6d 68 48 83 c4 08 5b 5d 41 5c 41 5d 41 5e 41 5f c3 66 2e 0f 1f 84 00 00 00 00 00 49 89 dc e9 68 ff f
 Segmentation fault

So update the logic to CFLAGS_append = " -std=gnu99"
to fix the issue.

(From OE-Core rev: 91359a91b8c89dc5e1f3a946137204156c47a3af)

Signed-off-by: Mingli Yu <Mingli.Yu@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-support/nettle/nettle_3.4.1.bb

@@ -30,7 +30,7 @@ inherit autotools ptest multilib_header
 EXTRA_AUTORECONF += "--exclude=aclocal"
 
 EXTRA_OECONF = "--disable-openssl"
-CFLAGS_append = " -std=c99"
+CFLAGS_append = " -std=gnu99"
 
 do_compile_ptest() {
         oe_runmake buildtest