package/pugixml: drop header-only option

Header-only option raises the following build failure with gerbera since
commit cc3a4a7b:

/home/thomas/autobuild/instance-2/output-1/host/lib/gcc/nios2-buildroot-linux-gnu/10.4.0/../../../../nios2-buildroot-linux-gnu/bin/ld: CMakeFiles/gerbera.dir/src/main.cc.o: in function `ConfigGenerator::~ConfigGenerator()':
/home/thomas/autobuild/instance-2/output-1/build/gerbera-1.10.0/src/config/config_generator.h:33: undefined reference to `pugi::xml_document::~xml_document()'

This build failure can't be fixed by adding a
!BR2_PACKAGE_PUGIXML_HEADER_ONLY dependency as it will create the
following recursive dependency:

package/gerbera/Config.in:1:error: recursive dependency detected!
package/gerbera/Config.in:1:    symbol BR2_PACKAGE_GERBERA depends on BR2_PACKAGE_PUGIXML_HEADER_ONLY
package/pugixml/Config.in:42:   symbol BR2_PACKAGE_PUGIXML_HEADER_ONLY depends on BR2_PACKAGE_PUGIXML
package/pugixml/Config.in:1:    symbol BR2_PACKAGE_PUGIXML is selected by BR2_PACKAGE_GERBERA

Moreover, commit 48b2e50e stated that
"Compact and header-only modes are not strictly needed for our use case,
but we did the work anyway and may be useful for someone else."

So dropping header-only seems to be the right approach

Fixes:
 - http://autobuild.buildroot.org/results/800537a1ef5f48d24c20aad7a9c96c56dfdc77f5



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>