libjq: extern C for C++

If using libjq from C++ it would be nice to not need to do this at the
import site, so just extern "C" to the public headers for libjq

Signed-off-by: Tyler Rockwood <rockwood@redpanda.com>

src/jv.h

@@ -5,6 +5,10 @@
 #include <stdint.h>
 #include <stdio.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #if (defined(__GNUC__) && __GNUC__ >= 7) || \
     (defined(__clang__) && __clang_major__ >= 10)
 # define JQ_FALLTHROUGH __attribute__((fallthrough))
@@ -260,6 +264,9 @@ int jv_cmp(jv, jv);
 jv jv_group(jv, jv);
 jv jv_sort(jv, jv);
 
+#ifdef __cplusplus
+}
+#endif
 
 #endif