libjson-c: update to 0.15

Drop patches as they've been upstreamed:
 * 001-Fix-CVE-2020-12762.patch

Refresh patches:
 * 000-libm.patch

Add patch to avoid build failure due to missing docs in tarball.

Signed-off-by: David Bauer <mail@david-bauer.net>

package/libs/libjson-c/patches/001-dont-build-docs.patch

@@ -0,0 +1,11 @@
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -401,8 +401,6 @@ set(JSON_C_SOURCES
+ include_directories(${PROJECT_SOURCE_DIR})
+ include_directories(${PROJECT_BINARY_DIR})
+ 
+-add_subdirectory(doc)
+-
+ # uninstall
+ add_custom_target(uninstall
+   COMMAND cat ${PROJECT_BINARY_DIR}/install_manifest.txt | xargs rm