Meson: drop -Werror-implicit-function-declaration

Implicit function declaration was removed in C99. GCC and CLang
have kept supporting it in C99 mode until GCC 14 and CLang 15.
There's little reason for enabling the relative -Werror.

This also fixes a warning when compiling C++ code:

  Compiling C++ object src/libcairo-2.dll.p/win32_cairo-dwrite-font.cpp.obj
  cc1plus.exe: warning: '-Werror=' argument '-Werror=implicit-function-declaration' is not valid for C++

References:

  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91092#c21

meson.build

@@ -44,7 +44,6 @@ cflags = []
 if cc.get_id() != 'msvc'
   cflags += [
     '-Wmissing-declarations',
-    '-Werror-implicit-function-declaration',
     '-Wpointer-arith',
     '-Wwrite-strings',
     '-Wsign-compare',