package/putty: fix static build

Fix the following static build failure raised since bump to version 0.78
in commit 5673ea3ce4d4dd721bb17fc8d5b1283f5c1080b4:

In file included from /home/buildroot/autobuild/instance-0/output-1/build/putty-0.78/putty.h:8,
                 from /home/buildroot/autobuild/instance-0/output-1/build/putty-0.78/callback.c:8:
/home/buildroot/autobuild/instance-0/output-1/build/putty-0.78/unix/platform.h:11:10: fatal error: dlfcn.h: No such file or directory
   11 | #include <dlfcn.h>                     /* Dynamic library loading */
      |          ^~~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/06f0b14bd0414f97b06070198e290fb3253348c5

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit 3d8e0a263f277ca113b78b1f283292c418528c11)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/putty/putty.mk

@@ -17,4 +17,8 @@ else ifeq ($(BR2_PACKAGE_LIBGTK2),y)
 PUTTY_DEPENDENCIES += libgtk2
 endif
 
+ifeq ($(BR2_STATIC_LIBS),y)
+PUTTY_CONF_OPTS += -DCMAKE_C_FLAGS="$(TARGET_CFLAGS) -DNO_LIBDL"
+endif
+
 $(eval $(cmake-package))