backend: Fix for missing _DYNAMIC declaration in musl libc

The _DYNAMIC symbol is not declared in musl link.h causing a build
error. Decalare it manually to fix this.

Signed-off-by: Naushir Patuck <naush@raspberrypi.com>

src/libpisp/backend/backend_default_config.cpp

@@ -20,6 +20,10 @@
 #include "common/pisp_pwl.hpp"
 #include "pisp_be_config.h"
 
+// musl doesn't declare _DYNAMIC in link.h, declare it manually, see:
+// https://github.com/raspberrypi/libpisp/issues/25
+extern ElfW(Dyn) _DYNAMIC[];
+
 using json = nlohmann::json;
 
 // Where it might be helpful we initialise some blocks with the "obvious" default parameters. This saves users the trouble,