libcurl: LICENSE value changes to SPDX

The curl license is a MIT/X derivative license, but
has a distinct identifier in SPDX, so use that:

https://spdx.org/licenses/curl.html

[Peter: reword commit message]
Signed-off-by: Naoki Matsumoto <n-matsumoto@melcoinc.co.jp>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

(cherry picked from commit d80110a635c23aac17dabf2e58b580470749a2c9)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libcurl/libcurl.mk

@@ -11,7 +11,7 @@ LIBCURL_DEPENDENCIES = host-pkgconf \
 	$(if $(BR2_PACKAGE_ZLIB),zlib) \
 	$(if $(BR2_PACKAGE_LIBIDN),libidn) \
 	$(if $(BR2_PACKAGE_RTMPDUMP),rtmpdump)
-LIBCURL_LICENSE = ISC
+LIBCURL_LICENSE = curl
 LIBCURL_LICENSE_FILES = COPYING
 LIBCURL_INSTALL_STAGING = YES