obs-filters: Actually define TRUE/FALSE

Doing something like '#if TRUE' when 'TRUE' isn't defined is the
equivalent to '#if 0' because the preprocessor evaluates an undefined
macro as 0/false.

plugins/obs-filters/obs-filters-config.h.in

 #pragma once
 
+#ifndef TRUE
+#define TRUE 1
+#endif
+
+#ifndef FALSE
+#define FALSE 0
+#endif
+
 #define SPEEXDSP_ENABLED @LIBSPEEXDSP_FOUND@