package/libcurl: drop _GNU_SOURCE workaround

This was needed for 8.14.0, but got fixed in 8.14.1 with:

https://github.com/curl/curl/issues/17512

So drop it again.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libcurl/libcurl.mk

@@ -14,7 +14,6 @@ LIBCURL_LICENSE = curl
 LIBCURL_LICENSE_FILES = COPYING
 LIBCURL_CPE_ID_VENDOR = haxx
 LIBCURL_INSTALL_STAGING = YES
-LIBCURL_CONF_ENV += CFLAGS="$(TARGET_CFLAGS) -D_GNU_SOURCE"
 
 # We disable NTLM delegation to winbinds ntlm_auth ('--disable-ntlm-wb')
 # support because it uses fork(), which doesn't work on non-MMU platforms.