bpf-headers: ship a modified version of stdarg.h from musl to fix ebpf build on glibc

Signed-off-by: Felix Fietkau <nbd@nbd.name>

package/kernel/bpf-headers/files/stdarg.h

@@ -0,0 +1,19 @@
+#ifndef _STDARG_H
+#define _STDARG_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef __builtin_va_list va_list;
+
+#define va_start(v,l)   __builtin_va_start(v,l)
+#define va_end(v)       __builtin_va_end(v)
+#define va_arg(v,l)     __builtin_va_arg(v,l)
+#define va_copy(d,s)    __builtin_va_copy(d,s)
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif