build: silence idiotic -Wformat-truncation

This warns about player/audio.c:253 with gcc 8.2.0. Although this
warning could be useful to check the worst case estimation, the compiler
doesn't explain how it gets its dumb, bogus result, so this is useless.
You'd just end up trying to make the compiler happy for no reason.

waftools/detections/compiler.py

@@ -42,7 +42,8 @@ def __add_generic_flags__(ctx):
                                  "-Wno-format-zero-length",
                                  "-Werror=format-security",
                                  "-Wno-redundant-decls",
-                                 "-Wvla"])
+                                 "-Wvla",
+                                 "-Wno-format-truncation"])
 
 def __add_gcc_flags__(ctx):
     ctx.env.CFLAGS += ["-Wall", "-Wundef", "-Wmissing-prototypes", "-Wshadow",