Add Argtable2 implicit patch

tvheadend/Dockerfile

@@ -127,6 +127,8 @@ RUN \
     /tmp/argtable --strip-components=1 && \
     cp /tmp/argtable2_config/config.* /tmp/argtable && \
     cd /tmp/argtable && \
+    cp /tmp/argtable2_config/implicit.patch /tmp/argtable && \
+    patch -p0 < implicit.patch && \
     ./configure \
     --prefix=/usr && \
     make -j 2 && \

tvheadend/argtable2_config/implicit.patch

@@ -0,0 +1,13 @@
+Fix:
+arg_int.c:60:12: error: implicitly declaring library function 'isspace' with type 'int (int)' [-Werror,-Wimplicit-function-declaration]
+arg_int.c:89:8: error: implicitly declaring library function 'toupper' with type 'int (int)' [-Werror,-Wimplicit-function-declaration]
+--- src/arg_int.c.orig	2011-01-10 04:44:01.000000000 -0600
++++ src/arg_int.c	2021-03-05 08:37:44.000000000 -0600
+@@ -24,6 +24,7 @@
+ #include "config.h"
+ #endif
+
++#include <ctype.h>
+ /* #ifdef HAVE_STDLIB_H */
+ #include <stdlib.h>
+ /* #endif */