build(linux): Suppress find error when GPAC is not installed

Redirect stderr to /dev/null for the GPAC source file search to avoid
showing "No such file or directory" error when GPAC is not installed.
The build continues to work correctly in both cases.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: cfsmp3

linux/build

@@ -40,7 +40,7 @@ BLD_INCLUDE="-I../src -I /usr/include/leptonica/ -I /usr/include/tesseract/ -I..
 SRC_LIBPNG="$(find ../src/thirdparty/libpng/ -name '*.c')"
 SRC_ZLIB="$(find ../src/thirdparty/zlib/ -name '*.c')"
 SRC_CCX="$(find ../src/lib_ccx/ -name '*.c')"
-SRC_GPAC="$(find /usr/include/gpac/ -name '*.c')"
+SRC_GPAC="$(find /usr/include/gpac/ -name '*.c' 2>/dev/null)"
 SRC_HASH="$(find ../src/thirdparty/lib_hash/ -name '*.c')"
 SRC_UTF8PROC="../src/thirdparty/utf8proc/utf8proc.c"
 SRC_FREETYPE="../src/thirdparty/freetype/autofit/autofit.c