Fix build after 7d3da7e9686df82bb595857e1fde0cf235ad5fba

Build is failing in Arch with
ninja: Entering directory `build'
[1/2] Compiling C object xdg-desktop-portal-hyprland.p/src_screencast_wlr_screencast.c.o
FAILED: xdg-desktop-portal-hyprland.p/src_screencast_wlr_screencast.c.o
cc -Ixdg-desktop-portal-hyprland.p -I. -I.. -I../include -Iprotocols -I/usr/include/pipewire-0.3 -I/usr/include/spa-0.2 -I/usr/include/libdrm -I/usr/include/uuid -fdiagnostics-color=always -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wextra -std=c11 -O0 -g -Wno-missing-braces -Wno-missing-field-initializers -Wno-unused-parameter -D_POSIX_C_SOURCE=200809L -D_GNU_SOURCE '-DSYSCONFDIR="/etc"' -DHAVE_LIBSYSTEMD=1 -D_REENTRANT -MD -MQ xdg-desktop-portal-hyprland.p/src_screencast_wlr_screencast.c.o -MF xdg-desktop-portal-hyprland.p/src_screencast_wlr_screencast.c.o.d -o xdg-desktop-portal-hyprland.p/src_screencast_wlr_screencast.c.o -c ../src/screencast/wlr_screencast.c
../src/screencast/wlr_screencast.c:617:19: error: static declaration of ‘vasprintf’ follows non-static declaration
  617 | static inline int vasprintf(char **strp, const char *fmt, va_list ap) {
      |                   ^~~~~~~~~
In file included from /usr/include/spa-0.2/spa/utils/defs.h:31,
                 from /usr/include/spa-0.2/spa/support/plugin.h:12,
                 from /usr/include/pipewire-0.3/pipewire/pipewire.h:12,
                 from ../include/screencast_common.h:5,
                 from ../include/wlr_screencast.h:4,
                 from ../src/screencast/wlr_screencast.c:1:
/usr/include/stdio.h:390:12: note: previous declaration of ‘vasprintf’ with type ‘int(char ** restrict,  const char * restrict,  __va_list_tag *)’
  390 | extern int vasprintf (char **__restrict __ptr, const char *__restrict __f,
      |            ^~~~~~~~~
ninja: build stopped: subcommand failed.
hyprwm / xdg-desktop-portal-hyprland

Fix build after 7d3da7e9686df82bb595857e1fde0cf235ad5fba #54
